# Exercises for Unit I I I (Basic Euclidean concepts and theorems)

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1 Exercises for Unit I I I (Basic Euclidean concepts and theorems) Default assumption: All points, etc. are assumed to lie in R 2 or R 3. I I I. : Perpendicular lines and planes Supplementary background readings. Ryan : pp. 6 8 Exercises to work.. Suppose that P, Q and T are three distinct planes, and suppose that they have at least one point in common but do not have a line in common. Prove that they have exactly one point in common. 2. Suppose P and Q are two planes which intersect in the line L = x + U, where the dimensional vector subspace U spanned by the unit vector u. Express these planes as translates of two dimensional subspaces, with P = x + V and Q = x + W. Let a and b be unit vectors in V and W respectively such that a and b are perpendicular (or normal) to u. Prove that the (cosine of the) angle (x + a) x (x + b) is equal to the (cosine of the) angle between the normals to P and Q; note that these normals are given by a u and b u. [ Hint : Express the dot product of the normals in terms of the dot product of a and b. Apply the formula for (v w) (y z) derived in Section I. 2. ] Note. If we let P(x + a) denotes the union of L with the set of all points on the same side of P as x + a, and we let Q(x + b) denotes the union of L with the set of all points on the same side of Q as x + b, then the union of P(x + a) and Q(x + b) is an example of a dihedral angle, and the result of the exercise states that two standard methods for defining the measure of this dihedral angle yield the same value.

2 3. Let X be a point in the plane P. Prove that there is a pair of perpendicular lines L and M in P which meet at X and that there is no line N in P through X which is perpendicular to both L and M. [ Hint : Try using linear algebra. ] 4. Assume the setting of the previous exercise, but also assume that P is contained in R 3. Prove that there is a unique line K through X which is perpendicular to both L and M. 5. (Ryan, Theorem 9, pp. 8 9) Let L and M be lines which intersect at Y, and for each X in L {Y}, let M X denote the foot of the unique perpendicular from X to M. Prove that for each positive real number a there are exactly two choices of X for which d(x, M X ) = a. [ Hint : Parametrize the line in the form Y + t V for some nonzero vector V, let W be a nonzero vector such that L and M lie in the plane determined by Y, Y + V, and Y + W with W perpendicular to V, and express d(x, M X ) in terms of t and the length of W. ] I I I.2 : Basic theorems on triangles Supplementary background readings. Ryan : pp Exercises to work.. (Review of topics from Section II.4) Suppose that we are given ABC and DEF, and let G and H denote the midpoints of [BC] and [EF] respectively. Prove that ABC DEF if and only if that GAB HDE. 2. Suppose that ABC is an isosceles triangle with d(a, B) = d(a, C), and D is a point of (BC) such that [AD bisects BAC. Prove that D is the midpoint of (BC) and that ADB = ADC = Suppose we are given isosceles PRL with d(r, P) = d(l, P). Let S and T be points on (RL) such that R S T, d(r, S) = d(l, T), and d(p, S) = d(p, T). Prove that RTP LSP and PSR = PTL. 4. Suppose we are given two lines AE and CD, and suppose that they meet at a point B which is the midpoint of [AE] and [CD]. Prove that AC DE.

3 5. Suppose that we are given lines AE, BD and FG which contain a common point C and also satisfy A F B, B C D, and D G E. Suppose also that d(a, C) = d(e, C) and d(b, C) = d(c, D). Prove that ABC EDC and AFC EGC. [ Hint : Part of the proof is to show that the betweenness properties A C E, and F C G suggested by the drawing are true. ] 6. Suppose that ABC is an isosceles triangle with d(a, B) = d(a, C), and let D and E be points of (AB) and (AC) respectively such that d(a, D) = d(a, E). Prove that BC DE. 7. Suppose that we are given ABC, and let D be a point in the interior of ABC such that [AD bisects CAB, [BD bisects CBA, and ADB = 30. Find the value of ACB. 8. Suppose that we are given points A, B, C such that A B C, and let DE AC such that D B E, CE AC, and DE AD. Prove that DAB = BEC. 9. (Ryan, Exercise 45, p. 70) Prove the following result due to Heron of Alexandria: Let P be a plane, let L be a line, let A and B be points on the same side of L in P, and let C be the mirror image of B with respect to L (formally, choose C such that L is the perpendicular bisector of [BC] ). Define a positive real valued function f on L by f(x) = d(a, X) + d(x, B). Then the minimum value of f(x) occurs when X lies on (AC). [Hint: Why is d(x, B) = d(x, C), and how is this relevant to the problem? ]

4 0. Given ABC, let X, Y and Z be points on the open segments (AB), (BC) and (AC) respectively. Prove that the sum of the lengths of the sides of ABC is greater than the sum of the lengths of the sides of XYZ.. Given ABC, let D and E be the midpoints of (BC) and (AC) respectively. Prove that d(d, E) = ½ d(a, B). 2. Given ABC, let D be the midpoint of (BC). Prove that d(a, D) < ½ [ d(a, B) + d(a, C) ]. [ Hint : Let F be the midpoint of (AB), and apply the previous exercise. ] 3. Given ABC, let X be a point in the interior of ABC. Prove that AXB + BXC + CXA = 360. [ Hint : There is one large triangle in the picture, and it is split into three smaller ones; the angle sum for each triangle is equal to 80. ] 4. Prove the Sloping Ladder Theorem: Suppose we are given right triangles ABC and DEF with right angles at C and F respectively such that the hypotenuses satisfy d(a, B) = d(d, E). If d(e, F) < d(b, C), then d(a, C) < d(d, F).

5 5. In ABC, one has d(a, C) < d(b, C). If E is the midpoint of [AB], is CEA acute (measurement less than 90 ) or obtuse (measurement greater than 90 )? Why? 6. Using the strong triangle inequality for noncollinear triples of points, determine which of the following triples cannot be the set of lengths for the sides of a triangle. (a), 2, 3 (b) 4, 5, 6 (c) 5, 5, (d) 5,, 8 7. Two sides of a triangle have lengths 0 and 5. Between what two numbers must the length of the third side lie? 8. Let n be a positive integer. Explain why there is a right triangle ABC with a right angle at C such that ( i ) the sides all have integral lengths, ( ii ) d(a, B) = n + and d(a, B) = n, provided the odd integer (2n + ) is a perfect square, and conclude that there are infinitely many values of n for which there is a right triangle ABC with right angle at C satisfying ( i ) and ( ii ). Find all n < 00 for which such triangles exist. [ Hint : Recall that the sum of the first k odd (positive) integers is equal to k 2. ] 9. Prove the Hinge Theorem: Given triangles ABC and ABD which satisfy d(a, C) = d(a, D), then d(b, C) < d(b, D) if and only if CAB < DAB. I I I.3 : Convex polygons Supplementary background readings. Ryan : pp. 76 8, 82 83

6 Exercises to work.. Suppose that A, B, C, D form the vertices of a convex quadrilateral, and let P, Q, R, S be the midpoints of [AB], [BC], [CD] and [DA] respectively. Prove that PQ RS and QR PS. [ Hint : In each case, the lines are parallel to one of the diagonals of the original convex quadrilateral.] 2. Suppose that A, B, C, D form the vertices of a convex quadrilateral, and let P, Q, R, S be the midpoints of [AB], [BC], [CD] and [DA] respectively. Prove that [PR] and [QS] meet at their common midpoint. [ Hint : Apply the preceding exercise.] 3. Suppose that A, B, C, D form the vertices of a parallelogram, and suppose that E and F are the midpoints of [AB] and [CD] respectively. Prove that E, B, F, D form the vertices of a parallelogram. [ Hint : There is a simple proof using vectors.] 4. Suppose that A, B, C, D form the vertices of a trapezoid, with AB CD, and assume that d(a, D) = d(c, D). Prove that [AC bisects DAB. 5. Suppose that A, B, C, D form the vertices of a parallelogram, and suppose that E and F are points of (BD) such that B F E and d(b, F) = d(d,e). Prove that AE CF. 6. A parallelogram is a rhombus if its four sides have equal length. Prove that a parallelogram is a rhombus if and only if its diagonals are perpendicular to each other. 7. Suppose that A, B, C, D form the vertices of a square, and let E be a point in the interior of the square such ABE is an equilateral triangle. Find EDC and ECD.

7 8. Prove a converse to Proposition I I I.3.: If A, B, C, D are coplanar points such that no three are collinear, then they form the vertices of a convex quadrilateral if the open diagonal segments (AC) and (BD) have a point in common. 9. Suppose that A, B, C, D are points in R 3 such that no three are collinear. Prove that they form the vertices of a convex quadrilateral if and only if D lies in the interior of ABC and D and B lie on opposite sides of AC. [ Hint : Recall that they form the vertices of a convex quadrilateral if and only if the open diagonal segments (AC) and (BD) have a point in common. ] 0. Suppose that A, B, C, D are points in R 2 such that no three are collinear, and express D as an affine combination D = xa + yb + zc, where x + y + z =. Using the preceding exercise, show that A, B, C, D form the vertices of a convex quadrilateral if and only if x and z are positive and y is negative.. Suppose that A, B, C, D are points in R 2 such that no three are collinear, and suppose that AB CD. Prove that A, B, C, D form the vertices of a convex quadrilateral if and only if C D is a positive multiple of B A (such a quadrilateral is a parallelogram if C D = B A and it is a trapezoid in the other cases). Standing hypotheses: In Exercises 7 below, points A, B, C, D in R 2 form the vertices of a convex quadrilateral such that AB CD. The lengths of C D and B A will be denoted by x and y respectively. 2. Prove that the line joining the midpoints of [AD] and [BC] is parallel to AB and CD, and its length is ½ (x + y). Also, prove that the line joining the midpoints of the diagonals [AC] and [BD] is parallel to AB and CD. 3. Suppose that x < y, and let E be the unique point on (AB such that d(a, E) = x. Prove that E lies on (AB) and AD CE (hence A, E, C, D form the vertices of a parallelogram). 4. Suppose again that x < y, and let E be as above. Prove that the following are equivalent: () d(a, D) = d(b, C) (2) DAB = CBA (3) ADC = BCD A trapezoid satisfying one (and hence all) of these conditions is called an isosceles trapezoid. 5. Suppose that A, B, C, D as above are the vertices of an isosceles trapezoid. Prove that the line joining the midpoints of [AB] and [CD] is the perpendicular bisector of these segments.

8 6. Suppose we are given an isosceles trapezoid as in the preceding exercise such that A = ( ½ y, 0), B = (½ y, 0), C = (½ x, h), and D = ( ½ x, h), where h > 0. Prove that the open diagonal segments (AC) and (BD) meet at a point (0, k) on the y axis, and express k/(h k) in terms of x and y. 7. Suppose that we are given A, B, C, D in R 2 whose coordinates are given by the equations A = (p, 0), B = (0, q), C = ( p, 0) and D = (0, q), where p, q > 0. (a) Prove that A, B, C, D form the vertices of a rhombus. [ Hint : First of all, show that d(a, B) = d(b, C) = d(c, D) = d(d, A). Next, note that A B = D C and use this to show that AB CD. Finally, modify the preceding step to show that AD BC. ] (b) Prove that the distance between the parallel lines AB and CD is equal to the distance between the parallel lines AD and BC. [ Hint : Let T be the orthogonal linear transformation defined by T (x, y) = (x, y), and view T as an isometry of R 2. What are the images of A, B, C, D under T? What are the images of the lines AB, AD, BC and CD under T? Using these conclusions, prove that if F AB and G CD are such that the line FG is perpendicular to both AB and CD, then T(F) AD and T(G) BC are such that the line T(F)T(G) is perpendicular to both AD and BC. Why will the result follow from this? ] 8. Given a square whose sides all have length a, it is possible to obtain a regular octagon by cutting away four isosceles right triangles at the edges as suggested by the figure below. Suppose that b is the length of the sides of the regular octagon constructed in this fashion. Express the value of b in terms of a. [ Hint : Let c be equal to the lengths of the legs of the isosceles right triangles that are removed to form the octagon. Find two equations relating a, b and c. There is a drawing on the next page.]

9 I I I.4 : Concurrence theorems Supplementary background readings. Ryan : pp Exercises to work.. (Ryan, Theorem 26, p. 54) Let ABC be a triangle in R 2. Define a real valued function g on R 2 by g(x) = d(x, A) 2 + d(x, B) 2 + d(x, C) 2. Prove that g(x) takes a minimum value when X is the centroid of ABC. 2. Suppose that the circumcenter V of ABC lies in the interior of that triangle. Prove that all three vertex angles of that triangle are acute (i.e., measure less than 90 ). [ Hint : Consider the three triangles VBC, VAB, VAC. Explain why they are isosceles, and show that VBC + VAB + VAC = 90. ] 3. Suppose we have a right triangle with a right angle at B, and let V be the midpoint of [AC]. Explain why d(v, B) = d(a, C), so that V is the circumcenter of the triangle. [ Hint: Using the final result in Section I.4 of the notes, show that the foot of the perpendicular from V to BC is the midpoint E of the segment [BC]. Why does this imply that VBC isosceles? ]

11 Exercises to work.. (Ryan, Exercise 40, p. 69) Prove that an affine transformation which preserves perpendicularity must be a similarity transformation. 2. Let T be a similarity transformation of R n with a ratio of similitude k which is not equal to. Prove there is a unique point z such that T(z) = z. [ Hint : Write T(z) = kaz + b, where A is given by an orthogonal matrix. Then the conclusion is equivalent to saying that there is a unique z such that (ka I)z = b. By linear algebra the latter happens if and only if there is no nonzero vector v such that kav = v. Assume to the contrary that such a vector exists, and using the orthogonality of A, explain why the length of the vector on the left side is equal to k v, and note that the length of the vector on the right side is just v. Why does this yield a contradiction? ] 3. Let ABC be a right triangle with a right angle at C such that d(a, C) = 3, d(b, C) = 4, and d(a, B) = 5. Let D be the point on (BC) such that [CD bisects ACB. Compute the distances d(a, D) and d(d, B). 4. Suppose we are given ABC, and let D (AB) be such that DCA = ABC. Prove that d(a, C) is the mean proportional between d(a, B) and d(a, D). 5. Let ABC be given, and let Z be a point on (AB. Let X and Y be points on the same side of AB as C such that AX, CZ and BY are all parallel to each other, and also assume that B C X and A C X. Prove that d( C, Z) = d( A, X ) +. d( B, Y )

12 6. Suppose that we are given positive real numbers a,, a n and b,, b n such that for all i. Prove that a b i i = a b a b + L+ a + L+ b n = n a b. 7. ( i ) Suppose that we are given ABC and DEF such that ABC ~ DEF with d(b, C) d(a, C) d(a, B). Prove that d(e, F) d(d, F) d(d, E). ( ii ) Suppose that we are given ABC with d(b, C) d(a, C) d(a, B) and that (A, B, C ) is a rearrangement of (A, B, C) such that ABC ~ k A B C. Prove that k =. [ Hint : Split the proof into cases depending upon whether ABC is equilateral, isosceles with the base shorter than the legs, isosceles with the legs shorter than the base, or scalene; i.e., no two sides have equal length. ] I I I.6 : Circles and classical constructions Exercises to work.. Let Γ be a circle with center Q, let [AB] and [CD] be chords of Γ (so that the endpoints lie on the circle), and let G and H be the midpoints of [AB] and [CD]. Prove that d(q, G) = d(q, H) if and only if d(a, B) = d(c, D), and d(q, G) < d(q,h) if and only if d(a, B) > d(c, D). 2. Let Γ be a circle with center Q, and let L be a line containing a point X on Γ. Prove that X is the only common point of Γ and L if and only if QX is perpendicular to L. (These are the usual synthetic descriptions for the tangent line to Γ at X.) [ Hint : If L also meets Γ at another point Y, explain why QXY is acute. ]

13 3. Let Γ be a circle with center Q, let X be a point in the exterior of Γ, and let A and B be two points of Γ which lie on opposite sides of QX such that XA and XB are tangent to Γ in the sense of the preceding exercise. Prove that d(x, A) = d(x, B). 4. Let Γ be a circle in the plane, let A be a point in the interior of Γ, and let X be a point different from A. Prove that the ray [AX meets Γ in exactly one point. [Hint : By the line circle theorem, the line AX meets Γ in two points B and C. Why do these points lie on opposite rays? ] 5. (SsA Congruence Theorem for Triangles) Suppose we have ABC and DEF such that CAB = FDE and d(b, C) = d(e, F) > d(a, B) = d(d, E). Prove that ABC DEF by supplying reasons for the steps listed below: () There is a point G (AC such that d(a, G) = d(e, F). (2) GAB FDE. (3) d(b, G) = d(e, F) = d(b, C). (4) G lies on the circle Γ with center B and radius d(b, C). (5) A lies in the interior of Γ. (6) (AG meets Γ in exactly one point. (7) C lies on (AG and Γ. (8) C = G. (9) ABC = ABG, and ABC DEF. 6. Let Γ be a circle whose center is Q, and let A be a point in the same plane that is not on Γ and not equal to Q. Prove that the distance from A to a point X on Γ is minimized for a point Y which also lies on the open ray (QA. [ Hint : There are two separate cases depending upon whether A is in the interior or exterior of the circle. In the first case the point Y satisfies Q A Y, and in the second case it satisfies Q Y A. Show first that if W is the other point on Γ QA, then the distance is not minimized at W; this leaves us with the cases where X does not lie on QA. The larger angle is opposite the greater side theorem is useful in the two separate cases when X does not lie on QA. ] 7. Let Γ and Γ 2 be concentric circles in the same plane, let Q be their center, and suppose that the radius p of Γ is less than the radius q of Γ 2. What is the set of all points X such that the shortest distance from X to Γ equals the shortest distance from X to Γ 2? Give a proof that your assertion is correct. 8. Prove the assertion in the notes about finding a triangle with given SAS data: Specifically, if we are given positive real numbers b and c, and α is a real number between 0 and 80, then there is a triangle ABC such that d(a, B) = c, d(a, C) = b, and CAB = α.

14 I I I.7 : Areas and volumes Exercises to work.. Prove that the area of the region bounded by a rhombus is equal to half the product of the lengths of its diagonals. 2. Using Heron s Formula, derive a formula for the area of the region bounded by an equilateral triangle whose sides all have length equal to a. 3. Is there a formula for the area of the region bounded by a convex quadrilateral in terms of the lengths of the four sides (and nothing else)? Give reasons for your answer. [ Footnote : Compare this with the formula of Brahmagupta which is stated in the notes and is valid if the vertices all lie on a circle. ] 4. Suppose that the radius of the circle inscribed in ABC is equal to r. Using Heron s Formula, prove that r is equal to sqrt( (s a)(s b)(s c)/ s ). [ Hint : Look at the drawing for Theorem I I I.4.8, and explain why this figure leads to a formula for the area of the triangle in terms of r and s. ]

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