# Foundations of Neutral Geometry

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1 C H A P T E R 12 Foundations of Neutral Geometry The play is independent of the pages on which it is printed, and pure geometries are independent of lecture rooms, or of any other detail of the physical world. G. H. Hardy in A Mathematician s Apology [10] ( ) 12.1 TRIANGLES AND PARALLELS As mentioned at the end of Chapter 11, Hilbert s incidence, betweenness, congruence, and Dedekind axioms form a basis for Euclidean, Hyperbolic, and Elliptic geometry. As such we say these axioms form a basis for Universal geometry. As shown in Chapter 11, the theorems of Universal geometry include Euclid s Propositions 1-15, 23, and ASA triangle congruence. Continuing from Proposition 15, there is another set of results that are not universal, but interestingly enough, are still common to two of our geometries Euclidean and Hyperbolic. Traditionally, the set of results common to these two geometries has been called Neutral or Absolute geometry Exterior Angle Theorem The first of Euclid s Propositions that does not hold in Universal geometry is the Exterior Angle Theorem (Proposition 16). 75

3 Foundations of Neutral Geometry 77 Then, CAD will be an exterior angle of triangle AB C that is congruent to a remote interior angle, which we just proved is impossible. Thus, CAD > ACB and a similar argument can be made to show CAD > ABC. On the face of it, this proof looks solid. It only uses results from earlier sections in this chapter. But, we know this theorem is false in Elliptic geometry, as we showed in chapter 8 that it is possible to construct a triangle with more than one right angle in Elliptic geometry. So, there must be some subtle assumption that we are making in this proof that is different in Euclidean and Hyperbolic geometry, as compared to Elliptic geometry. The first part of the proof involves a proof by contradiction. We start by assuming that one of the interior angles is congruent to an exterior angle. This leads to the conclusion that point D must be on BC, which is claimed to be a contradiction. However, this is only a contradiction if we are assuming that lines cannot double back on themselves. That is, if lines have infinite extent. However, this is not the case in Elliptic geometry. Here we have an illustration of a triangle in Elliptic geometry that has three right angles. In chapter 8 we saw that this is possible. In this case, it happens that the point D constructed so that AD = BC will be in exactly the same position as point B, so there is no contradiction! For Neutral geometry, we will assume the Exterior Angle Theorem holds. Thus, we are implicitly assuming that lines are not bounded, that they are of infinite extent. In Neutral geometry, we then have that Euclid s Propositions 1-15, 23, ASA, and the Exterior Angle Theorem (Proposition 16) all hold. Also, we have all of the results on betweenness, separation, congruence, angle and segment ordering, and numerical measure of segments and angles from Chapter 11. As we have not yet proven the circle-circle intersection property (section 11.10), we will be careful to not use this property (or Euclid s Proposition 1) in this section.

4 78 Exploring Geometry - Web Chapters Triangles Angles and Sides Let s continue on our review of Euclid s Propositions, assuming that we are working in Neutral geometry. Euclid s version of Proposition 17 states In any triangle two angles taken together in any manner are less than two right angles. We could use angle measure to translate what Euclid means by less than two right angles as a comparison to a given angle. However, we will prove the following Theorem, which is equivalent to Euclid s statement. Theorem (Proposition 17 substitute) In triangle ABC, for any pair of angles, say ABC and BAC, we have that ABC is less than the supplementary angle to BAC. The proof of this theorem follows directly from the Exterior Angle Theorem and is left as an exercise. Propositions 18 and 19 deal with relative comparisons of sides and angles in a triangle. Theorem (Proposition 18) In a triangle, the larger side is opposite the larger angle. Theorem (Proposition 19) In a triangle, the larger angle is opposite the larger side. The proofs of these two theorems are left as exercises. Proposition 20 has traditionally been called the Triangle Inequality. Theorem (Proposition 20) In triangle ABC, the sum of two sides of a triangle is always greater than the third side.

5 Foundations of Neutral Geometry 79 Proof: We have defined segment ordering and addition of segments in section Without loss of generality, let us assume that the two sides in question are AB and BC. We need to show that the sum of these segments is greater than AC. On the ray opposite to BA, there is a point D such that BD = BC (Axiom III-1 in section 11.5 ). Then, B is between A and D and AD is the sum of AB and BD. By exercise we can also say that AD is the sum of AB and BC. Since B is between A and D, then by Theorem and the definition of angle inequality (definition 11.20), we have that ACD > BCD. Since CBD is an isosceles triangle we have that BCD = BDC. Thus, by angle ordering (Theorem 11.38), we have that ACD > BDC. By Theorem 12.3 we than have that AD > AC. Since AD is the sum of AB and BC, then the sum of AB and BC is greater than AC. The triangle inequality can be used to prove some very useful results, such as the following: Theorem Given a line BC and a point A not on the line, the perpendicular AD from A to a point D on the line has the shortest distance among all segments from A that intersect BC. Proof: Let E be a point not equal to D on BC. A B D C We can find a point A on the ray opposite to DA such that A D = AD. Then, triangles ADE and A DE will be congruent by SAS. Also, AE + A E > AA by the Triangle Inequality Theorem. Thus, 2AE > 2AD and we are done. B D A A E C

6 80 Exploring Geometry - Web Chapters Continuing in our review of Euclid s propositions, we come to Proposition 21. This proposition deals with the relative comparison of side lengths between a triangle and a triangle created in its interior. Theorem (Proposition 21) Given triangle ABC, if we construct triangle DBC with D an interior point of ABC, then BD + CD < BA + CA. Also, BDC > BAC. Proof: Since D is interior to the triangle then BD will intersect side AC at some point E with A E C. In triangle ABE we have that AB + AE > BE by the Triangle Inequality theorem. B D A E C Then, AB+AE+EC > BE+EC. Since A E C we have AE+EC = AC and thus AB + AC > BE + EC. Similarly in triangle CDE we can get CE + ED > CD. Add BD to both sides and combine to get CE + BE > CD + BD. Then, by segment ordering we have that since AB +AC > BE +EC and CE + BE > CD + BD then AB + AC > CD + BD. The remaining statement about angle inequality is left as an exercise. Euclid s original version of Proposition 22 deals with the construction of a triangle from three given segments. It states that To construct a triangle from three lengths, it is necessary that when you add any pair of lengths, the sum is greater than the other length. This is a rather odd statement, as Euclid had already showed in Proposition 20 that if a triangle exists, then the Triangle Inequality holds. Thus, necessity had already been proven. In looking at Euclid s proof of Proposition 22, however, we see that he is really proving sufficiency of the conclusion. That is, if the sum of any two lengths is greater than the third, than one can construct a triangle with those three lengths. The next theorem will serve as an equivalent statement to Euclid s Proposition 22.

7 Foundations of Neutral Geometry 81 Theorem Given three segments a, b, and c, if the sum of any two of these segments is always greater than the third, then there is a triangle with sides congruent to a, b, and c. The proof of this theorem requires the circle-circle intersection property. We will prove this intersection property in the next section, and then we will return to the proof of this proposition. We will be careful not to use this proposition in the remaining proofs of this section. Continuing in our review of Euclid s axioms, we come to Proposition 23. As mentioned earlier in this section, this is part of Universal geometry, which we have already covered. Proposition 24 is another theorem dealing with triangle comparison. It is often called the Hinge Theorem. Theorem (Proposition 24) Let ABC and DEF be triangles with AB = DE and AC = DF. If CAB > F DE, then BC > EF. Proof: There are two cases to consider either AC and AB are congruent or they are not congruent. Assume that AC and AB are not congruent. We can assume that AC > AB. Since CAB > F DE, then we can find a ray AJ interior to CAB such that JAB = F DE (definition of angle order). Also, from Congruence Axiom III- 4 we can find a ray DG on the other side of DF from DE such that GDF = CAJ. By addition of angles, CAB = GDE.

8 82 Exploring Geometry - Web Chapters Now, using Congruence Axiom III-1, we can assume that G is the point on DG such that DF = DG. Then, DF G is an isosceles triangle and the angles at G and F in DF G are congruent. We now show that E, F, and G are non-collinear. By Theorem 12.4 we know BCA < CBA. Suppose E, F, and G were collinear. Then, by SAS congruence we would have that CAB = GDE, and thus, BCA = EGD and CBA = GED. This implies that EGD < GED. But, EGD = GF D. Thus, we have that the exterior angle GF D would be less than the opposite interior angle GED in DEF which is impossible. We conclude that E, F, and G are non-collinear. Consider EF G. We now show that GE is interior to F GD. Suppose that GE was exterior to F GD. By the Exterior Angle Theorem, DF G > DEF. By Theorem 12.4 we know that DEF > EF D. Thus, DF G > EF D. But, DF G = F GD. Thus, F GD > EF D. Since GE is exterior to F GD, then EGD > F GD, and so EGD > EF D. But this contradicts Theorem We can now assume that GE is interior to F GD. Then F GE < F GD. Also, since DF G = F GD, then F GE < DF G. But, DF G < EF G. Thus, F GE < EF G. By Theorem 12.4 we know that EG > EF. But, by SAS congruence, we know that CAB = GDE and so EG = BC. Thus, BC > EF.

9 Foundations of Neutral Geometry 83 The proof for the case where AC = AB is similar and is left as an exercise. Proposition 25 flips the concluding implication from Proposition 24. Theorem (Proposition 25) Let ABC and DEF be triangles with AB = DE and AC = DF. If BC > EF, then CAB > F DE. Proof: The proof relies on Proposition 24 and is left as an exercise. Proposition 26 consists of two triangle congruence results: ASA and AAS. ASA triangle congruence has already been proven when we covered the theorems of Universal geometry. We leave the proof of AAS as an exercise Transversals and Parallels Before we look at Propositions 27 and 28 we need to review some definitions concerning angles and parallels: Definition A line t is called transversal to two other lines l and m if t intersects both lines and the lines are not coincident. Let t be transversal to l and m, meeting l at A and m at A (Figure 12.1). Axiom I-3 guarantees that there is at least one other point on lines l and m and axiom II-2 guarantees that we can choose points B and C on l with B A C and B and C on m with B A C. Also, we can assume that B and B are on the same side of t, and C, C are also on the same side.

10 84 Exploring Geometry - Web Chapters Figure 12.1 Definition Angles CAA, C A A, BAA and B A A are called interior angles. (The angles having AA as a side) Also, CAA and B A A are called alternate interior angles as are C A A and BAA. All other angles formed are called exterior angles. Definition Pairs of angles, one interior and one exterior, on the same side of the transversal, are called corresponding angles. For example, in Figure 12.1, angles A AB and DA B are corresponding, as are EAC and AA C. Definition Two lines are parallel if they do not intersect. We can now consider Euclid s Proposition 27. Theorem (Alternate Interior Angle Theorem) If alternate interior angles are congruent then the lines are parallel, Proof: We are given that CAA = B A A as shown in Figure Assume that the two lines did meet at some point D. Without loss of generality, we can assume that D is on the same side of t as B and B.

11 Foundations of Neutral Geometry 85 l m E C C t A A B B D Figure 12.2 There is a point E on AC such that AE = A D. In Neutral geometry we assume that lines cannot double-back on themselves, and so we know that D E and we have two distinct triangles, A AE and AA D. By SAS A AE = AA D. Thus, DAA = EA A. Now, DAA and EAA are supplements. By Theorem we know that since EAA = DA A then DAA = C A A. But, the triangle congruence gave DAA = EA A. So, C A A = EA A. But axiom III-4 on the uniqueness of angles would then imply that E must be on m which is impossible, as then we would have two distinct lines intersecting in more than one point. The next two theorems make up Proposition 28. Theorem If two lines are cut by a transversal so that corresponding angles are congruent then the two lines are parallel. Proof: Exercise. Theorem If two lines are cut by a transversal so that the interior angles on the same side are supplementary then the two lines are parallel. Proof: Exercise. These two theorems have the following extremely useful corollaries. Corollary Two lines that are perpendicular to the same line are parallel.

12 86 Exploring Geometry - Web Chapters Proof: Suppose that lines l and m are both perpendicular to line t. Considering t as a transversal, then the alternate interior angles made are both right angles and hence congruent. Then, the lines are parallel by the Alternate Interior Angle Theorem. Note that this implies that the perpendicular dropped from a point to a line must be unique, since if there were two lines through the same point that were both perpendicular to the same line, then they would have to be parallel. Corollary Through a point there is only one line perpendicular to a given line. Corollary If l is a line and P is a point not on l, then there is at least one parallel to l through P. Proof: By the theorem above there is a unique perpendicular t through P to l. There is also a unique perpendicular m through P to t. Then, t is perpendicular to both l and m and so l and m are parallel. Note that this result is Proposition 31. Also note that the parallel through P is not necessarily unique. In fact, Hyperbolic geometry satisfies all of the axioms discussed so far in this chapter, and in this geometry there are multiple parallels through P. Exercise Prove Theorem Also, show that this theorem is equivalent to Euclid s version of Proposition 17. Exercise Prove Theorem That is, show that in any triangle, the greater angle lies opposite the greater side. [Hint: Figure 12.3] C A D B Figure 12.3 Exercise Prove Theorem That is, show that in any triangle

13 Foundations of Neutral Geometry 87 the greater side lies opposite the greater angle. [Hint: Assume triangle ABC has BCA > BAC. Then, either AB > BC or AB = BC or AB < BC. Show that two of these are impossible.] Exercise Prove the AAS triangle congruence result. That is, given triangles ABC and DEF if ABC = DEF, BCA = EF D and AC = DF, then ABC = DEF (Figure 12.4). C F A B D E Figure 12.4 [Hint: Suppose that BC and EF are not congruent. If EF < BC then there is a point G between B and C with GC = EF. Show that this leads to a contradiction of the exterior angle theorem.] Exercise Finish the proof of Theorem [Hint: use the Exterior Angle Theorem.] Exercise Finish the proof of Theorem [Hint: There are only two paragraphs in the proof that depend on the two segments AC and AB being non-congruent. Only a small modification in the argument is neded for each paragraph.] Exercise Prove Theorem [Hint: There are three cases to consider: either CAB < F DE, CAB = F DE, or CAB > F DE. Also, use Proposition 24.] Exercise Prove Theorem Hint: Suppose the lines intersect and find a contradiction. Exercise Prove Theorem Exercise Let ABC be a triangle with right angle ABC. Call the side opposite the right angle the hypotenuse and the other sides legs of the triangle. Show that both legs are less than the hypotenuse. Exercise Let ABC be a triangle with right angle ABC. On AB let D be a point with D A B. Show that BC < AC < DC.

14 88 Exploring Geometry - Web Chapters 12.2 CONTINUITY REDUX In Chapter 11 we discussed (without proof) the Circle-Circle continuity principle: (Circle-Circle Continuity) Given two circles c 1 and c 2, if c 1 contains a point inside of c 2 and also contains a point outside of c 2, then there are exactly two distinct points of c 1 that are also on c 2. ( We say they intersect in two points) There is also a Line-Circle continuity principle: (Line-Circle Continuity) Given a circle c and a line l, if l contains a point inside of c and also contains a point outside of c, then there are exactly two distinct points of c that are also on l. In this section we will show that in Neutral geometry, both of these continuity principles can be proven from Dedekind s axiom and the other results we have shown so far. The material in this section is technically dense, much like the material in section But, it is worth wading through, as it is crucial to a complete understanding of continuity principles in geometry. We will start with Line-Circle continuity. We first review some definitions regarding a circle. Definition The circle of radius AB and center O is the set of all points X such that OX = AB. A point P is said to be an interior point (or said to be inside the circle) if P = O or OP < AB. If OP > AB the point P is said to be an exterior point (or outside the circle). Theorem (Line-Circle Continuity) Given a circle c and a line l, if l contains a point inside of c and also contains a point outside of c, then there are exactly two distinct points of c that are also on l. Proof: Let O be the center of circle c, let AB be the radius, and let P be

15 Foundations of Neutral Geometry 89 the point inside c. There are two cases to consider. First, suppose that O is on l. Then, on the two rays on either side of O on l we know by congruence axiom III-1 that we can find unique points I 1 and I 2 with OI 1 = AB = OI2. Now, suppose that O is not on l. We can drop a perpendicular from O to l which intersects at some point N. Then, N must be an interior point of the circle. This is obvious if N = P. If N P then ONP is a right triangle and thus by exercise , we have that ON < OP and thus N is interior. We will create a partition for Dedekind s axiom l as follows. Let NT be a ray on l and let NS be the opposite ray (Figure 12.5). N T l S P O Figure 12.5 Let Σ 1 consist of NT together with all of the points W on l that are inside c (i.e., OW < AB). Let Σ 2 consist of the remaining points of l. Note that Σ 2 consists of points solely on NS. Clearly, Σ 1 is not empty. Also, by congruence axiom III-1, we know there is a point R on NS with NR = AB. Since ONR is a right triangle we have, by exercise , that OR > NR and R is in Σ 2. For the betweenness condition of Dedekind s axiom let Q 1, R 1 be two points of Σ 1 and Q 2, R 2 be points of Σ 2. Suppose that Q 1 Q 2 R 1. There are three cases to consider. First, if Q 1 and R 1 are both on NT, then Q 2 cannot be between these two points as it is on the opposite ray. Second, suppose only one of Q 1 and R 1 is on NT. We may assume that Q 1 is. Then, OR 1 < AB and R 1, Q 2 are both on the same side of N on l. Now, since Q 1 N Q 2 and Q 1 Q 2 R 1 then by four-point betweenness we have N Q 2 R 1. Since ONQ 2 is a right triangle and N Q 2 R 1 then, by exercise , we have OQ 2 < OR 1. But, OR 1 < AB, and so OQ 2 < AB which contradicts the fact that Q 2 is in

16 90 Exploring Geometry - Web Chapters Σ 2. Lastly, suppose both Q 1 and R 1 are on the ray NS with OQ 1 < AB and OR 1 < AB. Then, since Q 1 Q 2 R 1, we can use exercise to show that either OQ 2 < OQ 1 or OQ 2 < OR 1. In either case, OQ 2 < AB which contradicts Q 2 being in Σ 2. Now, suppose that Q 2 Q 1 R 2. This is impossible if Q 1 is on NT, as Q 2 and R 2 are on the opposite ray. So, OQ 1 < AB and Q 1 is on NS. Since Q 2 Q 1 R 2, then one of Q 2 and R 2 is on the same side of Q 1 as N. We can assume that Q 2 is on the same side of Q 1 as N. Then, either N Q 2 Q 1 or N Q 1 Q 2. We cannot have N Q 2 Q 1, as N is in Σ 1 and we have already shown that a point of Σ 2 cannot be between two points of Σ 1. Thus, we must have N Q 1 Q 2 (or Q 2 Q 1 N). From Q 2 Q 1 R 2, we conclude that N and R 2 are on the same side of Q 1. Thus, either Q 1 R 2 N or Q 1 N R 2. Again, we have shown above that Q 1 R 2 N is impossible. Since Q 1 and R 2 are both on NS, we cannot have Q 1 N R 2. Thus, by Dedekind s axiom there is a unique point X separating Σ 1 and Σ 2. We claim that OX = AB, and thus X is an intersection point of l with c. For suppose that OX < AB. By segment addition and congruence axiom II-1, we can find a point V on the vector opposite to XN with XV + OX = AB. (see figure 12.6) By the triangle inequality we have OV < XV + OX = AB. l V X P N O T Figure 12.6 Thus, V is in Σ 1. But, this would imply that X is between two points of Σ 1, namely V and N, which is impossible. A similar argument shows that OX > AB is not possible. Thus, OX = AB. It is left as an exercise to show that X must be on NS.

17 Foundations of Neutral Geometry 91 Will this intersection point be unique? Assume that there is another point of intersection X of ray NS with the circle. Let M be the midpoint of XX. Then, triangles OXM and OX M are congruent by SSS congruence and thus OMX = OMX and both must be right angles. But, we know by theorem and its corollaries that the perpendicular from O to l must be unique, which implies that N = M. But, this is impossible since if X and X are both on the same side of N, then all points on XX are on that side of N. Thus, X is unique. To finish the proof we need to find an intersection point along the ray NT. We leave this as an exercise. It is clear from the proof that we only used the point P that was inside the circle in our argument for the first intersection. We thus have the following: Corollary Given a circle c and a line l, if l contains a point P inside of c, then there are exactly two distinct points of c that are also on l. Also, the intersection points occur on each of the two opposite rays on l defined by P. The proof of the second statement of this corollary is left as an exercise. We also have the following result about segments and circles: Corollary Given a circle c and a segment P Q, if P is inside c and Q is outside c, then there is a point X interior to P Q that is also on c. Proof: Exercise. Before we prove the circle-circle continuity property, we will take a little side-trip to show that Dedekind s axiom can be extended to arcs of circles. Definition Let c be a circle with center O. If A B are two points on c then we call the segment AB a chord of the circle. If a chord passes through the center O we say it is a diameter of the circle.

18 92 Exploring Geometry - Web Chapters Definition A chord AB of a circle c will divide the points of a circle c into two parts, those points of c on one side of AB and those on the other side. Each of these two parts is called an open arc of the circle. An open arc determined by a diameter is called a semi-circle. If we include the endpoints A and B, we call the arc or semi-circle closed. Definition One of the two open arcs defined by a (nondiameter) chord AB will be within the angle AOB. This will be called a minor arc. The arc that is exterior to this angle is called a major arc. In order to extend Dedekind s axiom to arcs of circles we need to have a notion of betweenness for points on arcs. Definition Let AB be a chord of a circle c with center O and let σ be the minor arc of the chord. Let P, Q, R be points on σ. Then we say that R is between P and Q if the ray OR is between OP and OQ. (see figure 12.7) A P R Q O B c Figure 12.7 Theorem Dedekind s axiom and the Archimedean property can both be extended to minor arcs of circles.

19 Foundations of Neutral Geometry 93 Proof: The definition of betweenness for points on a minor arc is that of betweenness for rays in an angle. There is a direct correspondence between points on a minor arc and rays interior to the angle defined by the chord of the arc. What this correspondence allows us to do is to substitute any statement about points on an arc, including betweenness properties of such points, to statements about rays in an angle and betweenness properties of rays. Since we have already proven extensions of Dedekind s Axiom and the Archimedean property for rays within an angle, then we automatically have these same properties for points on a minor arc. We now return to the task of proving circle-circle continuity. We will need the following result for the proof. Lemma Let P be a point on circle c with center O. Let OR be perpendicular to OP at O. Then, for any number s > 0 there are points S 1 and S 2 on c such that P S 1 < s and P S 2 < s. Also, S 1 and S 2 are on opposite sides of OP and the same side of OR. Proof: By Theorem (Segment measure), there is a segment AB having length s. At P construct the perpendicular to OP. By congruence axiom III-1, we know there are points U and V on opposite sides of P on this perpendicular such that P U = AB = P V. By congruence axiom III-1, there are points S 1 and S 2 on OU and OV, respectively, such that S 1 and S 2 are on c. Also, OP U and OP V are right angles, so P OU and P OV are acute (Proposition 17). Then, P OS 1 < P OR. Let P OT be the supplementary angle to P OR. Then, P OS 2 < P OT (angle order properties with supplementary angles). Thus, S 1 and S 2 are on opposite sides of OP and the same side of OR. Now, OP S 1 is an isosceles triangle. Thus, OS 1 P must be acute (Proposition 17). Then, P S 1 U is obtuse, by Theorem In triangle P S 1 U we than have P US 1 is acute (Proposition 17). Thus, by

20 94 Exploring Geometry - Web Chapters Theorem 12.3, we have that P S 1 < P U. Since the length of P U is s, we have P S 1 < s. A similar argument shows that P S 2 < s. We will also use the following result to put circle-circle continuity into a standard configuration. Theorem Let c 1, c 2 be two circles with centers O 1, O 2. Suppose there is a point P on c 1 that is inside c 2 and there is a point Q on c 1 that is outside c 2. Then, the line through the centers O 1, O 2 must meet circle c 1 in two points I 1 and I 2 with one of these points inside c 2 and the other outside c 2. Proof: For our analysis, we will use the segment measure properties we covered in Chapter 11. Specifically, we will let r 1 be the measure of the radius for c 1, and r 2 the measure for the radius of c 2. Since O 1 is inside of c 1 then by exercise we have that O 1 O 2 intersects c 1 in two points I 1 and I 2. There are three cases to consider for point I 1. Either I 1 O 1 O 2 or O 1 I 1 O 2 or O 1 O 2 I 1. Case (1): I 1 O 1 O 2. Then, I 1 O 2 = I1 O 1 + O 1 O 2, or I 1 O 2 = O 1 O 2 + r 1. Also, by the triangle inequality for O 1 QO 2 we have O 2 Q < O 1 O 2 + r 1. Since Q is outside c 2, we have O 2 Q > r 2. Then, r 2 < (O 1 O 2 + r 1 ) (= I 1 O 2 ). Thus, I 1 is outside c 2. P Q I O O I c 1 c 2 Case (2): O 1 I 1 O 2. Then, O 1 O 2 = O 1 I 1 + I 1 O 2 = r 1 + I 1 O 2. Also, by triangle inequality for O 2 P O 1 we have O 1 O 2 < r 1 + O 2 P. Thus, r 1 + I 1 O 2 < r 1 + O 2 P and so I 1 O 2 < O 2 P. Since O 2 P < r 2 we have that I 1 O 2 < r 2 and I 1 must lie inside c 2. P Q I O I c 1 O 2 c 2 Case (3): O 1 O 2 I 1. A similar argument to case (2) shows that I 1 must lie inside of c 2 (exercise). What about I 2? In Case (1), we have I 1 O 1 O 2. Also, I 1 O 1 I 2,

21 Foundations of Neutral Geometry 95 as I 1 and I 2 must be on opposite sides of O 1 (by the work we did in the proof of Theorem 12.17). Thus, O 2 and I 2 are on the same side of O 1 and we have either O 1 I 2 O 2 or O 1 O 2 I 2, In both cases, we can replace I 1 by I 2 in the work above. By doing so, we would either be in Case (2) or (3) for I 2, and so I 2 must be inside c 2. Now consider Case (2) or (3) for I 1. We have O 1 I 1 O 2 or O 1 O 2 I 1. Again we have I 1 O 1 I 2. By four-point betweenness, in either case we get that I 2 O 1 O 2. Thus, for I 2, we would be in Case (1) and I 2 must be outside c 2. We are now ready to prove circle-circle continuity. Theorem (Circle-Circle Continuity) Let c 1, c 2 be two circles with centers O 1, O 2. Suppose that c 1 has one point P inside and one point Q outside of c 2. Then, the two circles intersect in two points. Proof: By Theorem we can assume there is a diameter I 1 I 2 of c 1 with I 1 outside c 2 and I 2 inside c 2 (Figure 12.8). Figure 12.8 By Theorem 11.47, we can construct a perpendicular O 1 R to I 1 I 2 at O 1. Let O 1 R be one of the two rays defined. Then, by congruence axiom III-1 we can assume R is a point such that the measure of O 1 R is r 1, where r 1 is the measure of the radius of c 1. That is, R is on c 1. Now, R is either outside c 2, inside c 2, or on c 2 (exercise). If it is on c 2, we have

22 96 Exploring Geometry - Web Chapters found an intersection point for the two circles. Let this point be M for reference later in this proof. We note that M is on one side of I 1 I 2. If R is inside c 2, then the points I 1 and R form a pair of inside-outside points to c 2. If R is outside c 2 then the points R and I 2 form a pair of inside-outside points. In either case, we have a well-defined angle, with vertex at O 1, with sides being two rays intersecting c 1 at points inside and outside c 2. Without loss of generality we assume that R is outside c 2 and consider angle I 2 O 1 R. Since I 2 is inside c 2, then O 2 I 2 < r 2. Let s = r 2 O 2 I 2. Using Lemma for c 1 and the number s, we have that there 2 is a point S interior to arc σ = arc(i 2 R) such that I 2 S < s 2. Likewise, since R is outside c 2 then O 2 R > r 2. Let s = O 2 R r 2. Then, there is a point S on this arc such that RS < s 2. By the triangle inequality, we have that O 2 S < I 2 S + O 2 I 2. Since I 2 S < s 2 < s < r 2 O 2 I 2, we have O 2 S < r 2 O 2 I 2 + O 2 I 2 = r 2. Thus, S is inside c 2. Again, by the triangle inequality, we have that O 2 R < RS + O 2 S. Since RS < s 2 < s, then O 2 S > O 2 R RS > O 2 R s 2 > O 2R s = r 2. Thus, S is outside c 2. Define a Dedekind cut for the rays within I 2 O 1 R as follows. Let Σ 1 consist of rays with points on σ inside circle c 2. Let Σ 2 consist of rays with points on σ that are on or outside circle c 2. Clearly, these two sets are non-empty. We now show these sets satisfy the betweenness property for Dedekind s axiom for arcs. Suppose we had a ray O 1 Q 1 of Σ 1 that was between two rays O 1 Q 2 and O 1 R 2 of Σ 2.

23 Then, I 2 O 1 Q 1 is greater than I 2 O 1 R 2 (or I 2 O 1 Q 2 ). In either case, we can show, using Theorem 12.9 that this implies that O 2 Q 1 > O 2 R 2 (or O 2 Q 1 > O 2 Q 2 ) (exercise). But, this would mean that Q 1 is outside c 2, which contradicts Q 1 being in Σ 1. Foundations of Neutral Geometry 97 Similarly, we cannot have a point of Σ 2 between two points of Σ 1. Dedekind s property for angles (Theorem 11.56) requires a partition of rays (which we have), a betweenness condition (shown above), and the condition that both subsets of the partition contain at least one interior ray to the angle. These are O 1 S and O 1 S defined above. Thus, there is a unique ray O 1 M interior to I 2 O 1 R separating Σ 1 and Σ 2. We need to show that O 2 M = r 2, where r 2 is the radius of c 2. Suppose O 2 M > r 2. Let t = O 2 M r 2. Let O 1 U be perpendicular to OM at O. We can choose U so that U is on the other side of O 1 O 2 as M. By Lemma 12.21, there is a point T on c 1 such that T is interior to UO 1 M and MT < t 2. Now, since I 2 O 1 M is within and less than UO 1 M, it may be that T is exterior to I 2 O 1 M. If this is the case, then we replace T by the point T that is on c 1 and on the bisector of I 2 O 1 M. Since T O 1 M < T O 1 M we can show, using Theorem 12.9 that this implies that T M < T M (exercise). Then, for T we have MT < MT < t 2. We conclude that if O 2 M > r 2, then for t = O 2 M r 2, there is a point T interior to I 2 O 1 M such that MT < t. By the triangle 2 inequality, we have that O 2 M < MT + O 2 T. Since MT < t 2 < t, then O 2 T > O 2 M MT > O 2 M t 2 > O 2M t = r 2. Thus, T is outside c 2. But, this implies that M is between two points, R and T, that are both outside c 2. This contradicts Dedekind s property for rays.

24 98 Exploring Geometry - Web Chapters A similar argument shows that O 2 M < r 2 is impossible. Thus, O 2 M = r 2 and M is on c 2. To find the second intersection point, we can copy angle I 2 O 1 M to the angle on the other side of O 1 O 2, by congruence axiom III-4, yielding an angle I 2 O 1 M. By congruence axiom III-1 we can assume that O 1 M = O 1 M. This implies that M is on c 1. Then, O 2 O 1 M = O 2 O 1 M by SAS triangle congruence. Thus, O 2 M = O 2 M, which means that M is on c 2. We can now complete the proof Euclid s Proposition 22, which we had to leave dangling in the last section. Theorem Given three segments a, b, and c, if the sum of any two of these segments is always greater than the third, then there is a triangle with sides congruent to a, b, and c. Proof: By repeated pair-wise comparison of the three segments, using segment ordering, we can assume that either 1) all segments are pairwise non-congruent, 2) two of the three are congruent, or 3) all three are pair-wise congruent. If all three segments are pair-wise non-congruent, then we can assume that a > b > c. Let l be a line and A a point on l. By Axiom III-1 in section 11.5 there is a point A on l such that segment a is congruent to AA. Since a > b then there is a point B between A and A such that AB is congruent to b. Also, there is a point B on the other side of A from B such that AB is congruent to b. Since a > c there is a point C between A and A such that A C is congruent to c. Also, there is a point C on the other side of A from C such that A C is congruent to c. Since b + c > a, then AB + A C > AA. Since B and C are between A and A, then, by exercise we know that C is between A and B and B is between C and A.

25 Foundations of Neutral Geometry 99 Construct the circle c centered at A of radius AB and the circle c centered at A of radius A C. Since C is on AB, then AC < AB and C is interior to circle c. In Neutral geometry, lines do not return upon themselves, and so C is not on AB. Thus, AC > AB and C is exterior to circle c. So, by the circle-circle continuity principle, circles c and c must intersect at two points D and D. Then, triangle ADA will have sides congruent to a, b, and c. The case where two of a, b, and c are congruent can be argued in a similar fashion. The proof is left as an exercise. The case where a, b, and c are all congruent is equivalent to the construction of an equilateral triangle, which was proven in Chapter!11. This finishes our tour of Neutral geometry. The observant (and patient) reader will see that we have now proven all of the first 28 Propositions of Book I of Elements, plus Proposition 31. Euclid s Proposition 29 deals with the properties of a line crossing two parallel lines. The proof of this proposition relies on the uniqueness of parallels, and thus is outside of Neutral geometry. This is the first of Euclid s results that is uniquely a property of Euclidean geometry. Exercise In the proof of Theorem (Line-Circle Continuity) show that the point X given by Dedekind s axiom must be on NP. Exercise In the proof of Theorem (Line-Circle Continuity) show that there is a second intersection point of l with c on NT. Exercise Finish the proof of Corollary [Hint: In the proof of Theorem we showed that the intersection points were on opposite rays NS and NT, where N was interior to the circle and both of these rays were incident on l. There were two cases for N. Use these as the basis for your proof.] Exercise Prove Corollary [Hint: See the hint for exercise Also, make ample use of four-point betweenness.]

26 100 Exploring Geometry - Web Chapters Exercise Finish the proof of Theorem That is, given three segments a, b, and c, where exactly two of the three are congruent, show there is a triangle with sides congruent to these three segments. [Hint: Follow the outline of the first part of the proof of Theorem 12.24, but use circles based on the two segments of differing lengths.] Exercise Let C be a circle and P a point in the plane. Show that P is either on the circle, outside the circle, or inside the circle.

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