All Hail Euler the ruler!

Geometry Level 4

Consider a triangle Δ A B C \Delta ABC whose circumcenter is at the origin. If in Δ A B C \Delta ABC , the coordinates of the centroid G G are ( x G , y G ) \left(x_G,y_G\right) and the coordinates of the orthocenter H H are ( x H , y H ) \left(x_H,y_H\right) .

Find the ratio x H y H x G y G \dfrac{x_Hy_H}{x_Gy_G} .

Bonus : Don't forget the lonely incenter!


The answer is 9.000.

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2 solutions

OGH is a st. line. We know OG:OH::1:3.... O(0,0). Let G(m, n) then H(3m, 3n).
So X H Y H X G Y G = 3 m 3 n m n = 9 \dfrac {X_H*Y_H} {X_G*Y_G}= \dfrac {3m*3n} {m*n}= \large ~~~\color{#D61F06}{9 }

Nihar Mahajan
Jun 10, 2015

Let C = ( x C , y C ) = ( 0 , 0 ) C=(x_C,y_C)=(0,0) .

We know that G H G O = 2 1 \dfrac{GH}{GO}=\dfrac{2}{1} .So we will use section formula , where m = 2 , n = 1 m=2,n=1 .

( x G , y G ) = ( m x C + n x H m + n , m y C + n y H m + n ) = ( x H 3 , y H 3 ) s i n c e ( x C , y C ) = ( 0 , 0 ) r a t i o = x H y H x G y G = x H y H ( x H 3 ) ( y H 3 ) = 9 (x_G,y_G)=\left(\dfrac{mx_C+nx_H}{m+n} ,\dfrac{my_C+ny_H}{m+n}\right)= \left(\dfrac{x_H}{3},\dfrac{y_H}{3}\right) \\ \dots \ since \ (x_C,y_C)=(0,0) \\ \Rightarrow \ ratio \ = \dfrac{x_Hy_H}{x_Gy_G} = \dfrac{x_Hy_H}{\left(\dfrac{x_H}{3}\right)\left(\dfrac{y_H}{3}\right)} = \huge\boxed{9}

Bonus: Similarly using section formula , we can find the coordinates of incenter too.Try it!

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