Non-Isosceles Centers

Geometry Level 5

I'm thinking of a scalene triangle in which one side is length 6. If both the barycenter and orthocenter lie on the triangle's incircle, what is the maximum distance between the barycenter and orthocenter? If this distance is d d , submit 1 0 4 × d 10^4 \times d rounded to the nearest integer.


The answer is 45790.

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1 solution

Mark Hennings
Feb 9, 2021

This is done by number-crunching. I shall use the u v w uvw -representation of a triangle. Any triangle has sides of the form a = v + w a=v+w , b = u + w b=u+w , c = u + v c=u+v for u , v > 0 u,v > 0 . The semiperimeter, area, inradius and circumradius of the triangle A B C ABC can be written in terms of u , v , w u,v,w as follows: s = u + v + w Δ = u v w ( u + v + w ) r = u v w u + v + w R = a b c 4 Δ = ( u + v ) ( u + w ) ( v + w ) 4 u v w ( u + v + w ) \begin{array}{rclcrcl} s & = & u+v+w & \hspace{2cm} & \Delta & = & \sqrt{uvw(u+v+w)} \\[2ex] r & = & \sqrt{\frac{uvw}{u+v+w}} & & R & = & \frac{abc}{4\Delta} \; =\; \frac{(u+v)(u+w)(v+w)}{4\sqrt{uvw(u+v+w)}} \end{array} and we know that the distances between the orthocentre H H , the incentre I I and the centroid G G are given by I H = 2 r 2 + 4 R 2 1 2 ( a 2 + b 2 + c 2 ) = 2 r 2 + 4 R 2 ( u 2 + v 2 + w 2 + u v + u w + v w ) I G = 1 3 5 r 2 16 r R + s 2 G H = 2 3 9 R 2 ( a 2 + b 2 + c 2 ) = 2 3 9 R 2 2 ( u 2 + v 2 + w 2 + u v + u w + v w ) \begin{aligned} IH & = \; \sqrt{2r^2 + 4R^2 - \tfrac12(a^2+b^2+c^2)} \; = \; \sqrt{2r^2 + 4R^2 - (u^2+v^2+w^2+uv+uw+vw)} \\ IG & = \; \tfrac13\sqrt{5r^2 - 16rR + s^2} \\ GH & = \; \tfrac23\sqrt{9R^2 - (a^2+b^2+c^2)} \; = \; \tfrac23\sqrt{9R^2 - 2(u^2+v^2+w^2+uv+uw+vw)} \end{aligned} We want to solve the equations I G = I H = r v + w = 6 IG \; = \; IH \; = \; r \hspace{2cm} v + w = 6 over the range u > 0 u > 0 and 0 < v < 6 0 < v < 6 . For definiteness, we assume that v w v \le w , and so we solve over u > 0 u > 0 and 0 < v 3 0 < v \le 3 . Solving these equations numerically yields three possible solutions: ( u , v , w ) = ( 1.0421399174810988 , 1.310353335593083 , 4.689646664406917 ) ( 1.3716700545694032 , 1.09090584856126 , 4.90909415143874 ) ( 11.960875955614997 , 2.6579627791559095 , 3.3420372208440905 ) \begin{aligned} (u,v,w) \; = \; & (1.0421399174810988,1.310353335593083,4.689646664406917)\\ & (1.3716700545694032,1.09090584856126,4.90909415143874)\\ & (11.960875955614997,2.6579627791559095,3.3420372208440905) \end{aligned} which give the following values of G H GH : 1.7953426074814323 1.8793539310984508 4.578995340714356 1.7953426074814323 \hspace{0.5cm} 1.8793539310984508 \hspace{0.5cm} 4.578995340714356 respectively. This makes the last solution of the three the one we want. The desired triangle has sides a = 6 a=6 , b = 15.302913176459088 b=15.302913176459088 and c = 14.618838734770906 c=14.618838734770906 , and d = G H = 4.578995340714356 d = GH = 4.578995340714356 , so that 1 0 4 d = 45789.95340714356 10^4d = 45789.95340714356 , which rounds to 45790 \boxed{45790} .

I was wondering if you would find a way to solve this. Good job. Thank you for pointing out u v w -triangles uvw\text{-triangles} . That looks handy for future problems.

Fletcher Mattox - 4 months ago

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Note that u = s a , v = s b , w = s c u=s-a,v=s-b,w=s-c , so u , v , w u,v,w are the distances that the contact points of the incircle split the triangle sides into. The nice thing about u , v , w u,v,w is that any set of positive u , v , w u,v,w defines a triangle, which means that there are no domain conditions (like c < a + b c < a+b ) to handle when solving numeric problems.

Mark Hennings - 4 months ago

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