Collinearity of the First Trisection Points of Cevian Segments

Transcription

1 Forum eometricorum Volume 11 (2011) FORUM EOM ISSN ollinearity of the First Trisection oints of evian Segments Francisco Javier arcía apitán bstract. We show that the first trisection points of the cevian segments of a finite point are collinear if and only if lies on the Steiner circum-ellipse. Some further results are obtained concerning the line containing these first trisection points. This note answers a question of aul Yiu []: In the plane of a given triangle, what is the locus of for which the first trisection points of the three cevian segments (the ones nearer to the vertices) are collinear? We identify this locus and establish some further results. Let = (u : v : w) be in homogeneous barycentric coordinates with respect to triangle. Its cevian triangle have vertices = (0 : v : w), = (u : 0 : w), = (u : v : 0). The first trisection points of the cevian segments,, are the points dividing these segments in the ratio 1 : 2. These are the points X = (2(v+w) : v : w), Y = (u : 2(w+u) : w), Z = (u : v : 2(u+v)). These three points are collinear if and only if 2(v + w) v w 0 = u 2(w + u) w = 6(u + v + w)(uv + vw + wu). u v 2(u + v) It follows that, for a finite point, the first trisection points are collinear if and only if lies on the Steiner circum-ellipse uv + vw + wu = 0. roposition 1. If = (u : v : w) lies on the Steiner circum-ellipse, the line L containing the three first trisection points of the three cevian segments,, has equation v 1 ) x + w w 1 ) y + u u 1 ) z = 0. v ublication Date: November 10, ommunicating Editor: aul Yiu.

2 218 F. J. arcía apitán roof. The line L contains the point X since v 1 ) 2(v + w) + w w 1 ) v + u u 1 ) w v (v w)(uv + vw + wu) = uvw = 0; similarly for Y and Z. X Y Z Figure 1. L Let be a point on the Steiner circum-ellipse, with cevian triangle. onstruct points,, on the lines,, respectively such that =, =, =. The lines,, are parallel, and their common infinite point is the isotomic conjugate of. It is clear from roposition 1 that the line L contains the isotomic conjugate of : v 1 w ) 1 u + w 1 ) 1 u v + u 1 ) 1 v w = 0. It follows that L is parallel to,, and ; see Figure 1.

3 ollinearity of first trisection points of cevian segments 219 orollary 2. iven a line l containing the centroid, there is a unique point on the Steiner circum-ellipse such that L = l, namely, is the isotomic conjugate of the infinite point of the line l. Lemma. Let l : px+qy+rz = 0 be a line through the centroid. The conjugate diameter in the Steiner circum-ellipse is the line (q r)x+(r p)y +(p q)z = 0. roof. The parallel of l through intersects the ellipse again at the point ( ) 1 N = q r : 1 p q : 1. r p The midpoint of N is the point (2(r p)(p q) (q r) 2 : (q r)(r p) : (q r)(p q)). This point lies on the line (q r)x + (r p)y + (p q)z = 0, which also contains the centroid. This line is therefore the conjugate diameter of l. orollary ( 4. Let) l : px + qy + rz = 0 be a line through the centroid, so that l := 1 p : 1 q : 1 r is a point on the Steiner circum-ellipse. The conjugate diameter of l in the ellipse is the line L l. roposition 5. Let and Q be points on the Steiner circum-ellipse. The lines L and L Q are conjugate diameters if and only if and Q are antipodal. roof. Suppose L has equation px+qy+rz = 0 with p+q+r = 0. The line L Q is the conjugate diameter of L if and only if its ( has equation (q( r)x+(r p)y+ ) (p q)z = 0. Thus, and Q are the points 1 p : 1 q r) : 1 and 1 q r : 1 r p : 1 p q. These are antipodal points since the line joining them, namely, contains the centroid. p(q r)x + q(r p)y + r(p q)z = 0, It is easy to determine the line through whose intersections with the Steiner circum-ellipse are two points and Q such that L : px + qy + rz = 0 and L Q : (q r)x + (r p)y + (p q)z = 0 are the axes of the ellipse. This is the case if the two conjugate axes are perpendicular. Now, the infinite points of the lines L and L Q are respectively q r : r p : p q and p : q : r. They are perpendicular, according to [4, 4.5, Theorem] if and only if (b 2 + c 2 a 2 )p(q r) + (b 2 + c 2 a 2 )q(r p) + (a 2 + b 2 c 2 )r(p q) = 0. Equivalently, (b 2 +c 2 a 2 ) or r 1 ) +(c 2 +a 2 b 2 ) q p 1 ) +(a 2 +b 2 c 2 ) r q 1 ) = 0, p b 2 c 2 p + c2 a 2 q + a2 b 2 r = 0.

4 220 F. J. arcía apitán ( ) Therefore, = 1 p : 1 q : 1 r is an intersection of the Steiner circum-ellipse and the line (b 2 c 2 )x + (c 2 a 2 )y + (a 2 b 2 )z = 0, which contains the centroid and the symmedian point K = (a 2 : b 2 : c 2 ). L Q Y L Z Q X X Q K Q Q Y Q Z Q Figure 2. point on the line K has homogeneous barycentric coordinates (a 2 + t : b 2 + t : c 2 + t) for some t. If this point lies on the Steiner circum-ellipse, then From this, where (b 2 + t)(c 2 + t) + (c 2 + t)(a 2 + t) + (a 2 + t)(b 2 + t) = 0. t = 1 (a2 + b 2 + c 2 ± D), D = a 4 + b 4 + c 4 a 2 b 2 b 2 c 2 c 2 a 2. We obtain, with ε = ±1, the two points and Q (b 2 + c 2 2a 2 + ε D : c 2 + a 2 2b 2 + ε D : a 2 + b 2 2c 2 + ε D) for which L and L Q are the axes of the Steiner circum-ellipse. Their isotomic conjugates are the points X 41 and X 414 in [2], which are the infinite points of the axes of the ellipse. We conclude the present note with two remarks.

5 ollinearity of first trisection points of cevian segments 221 X Y Z Figure. Remarks. (1) The line L contains the centroids of triangle and the cevian triangle of. roof. learly L contains the centroid = (1 : 1 : 1). Since X, Y, Z are on the line L, so is the centroid = 1 (X + Y + Z) of the degenerate triangle XY Z. Since X =, Y =, Z =, it follows that L also contains the point 1 ( + + ), the centroid of the cevian triangle of. (2) More generally, if we consider X, Y, Z dividing the cevian segments,, in the ratio m : n, these points are collinear if and only if n(n m)(u + v + w)(uv + vw + wu) + (2m n)(m + n)uvw = 0. In particular, for the second trisection points, we take m = 2, n = 1 and obtain for the locus of the cubic u(v 2 + w 2 ) + v(w 2 + u 2 ) + w(u 2 + v 2 ) 6uvw = 0, which is the Tucker nodal cubic K015 in ibert s catalogue of cubic curves [1]. References [1]. ibert, ubics in the Triangle lane, available from [2]. Kimberling, Encyclopedia of Triangle enters, available at []. Yiu, Hyacinthos message 227, January 1, [4]. Yiu, Introduction to the eometry of the Triangle, Florida tlantic University Lecture Notes, Francisco Javier arcía apitán: Departamento de Matemáticas, I.E.S. lvarez ubero, vda. residente lcalá-zamora, s/n, riego de órdoba, órdoba, Spain address: garciacapitan@gmail.com