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Geometry Level 4

Let A B C ABC be a triangle in which A B = A C AB=AC . Suppose the orthocenter of the triangle lies on the incircle. If the value of A B B C \dfrac{AB}{BC} can be expressed as a b \dfrac ab , where a a and b b are coprime positive integers , entre your answer as a + b a+b .


The answer is 7.

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3 solutions

Construct points D , E , F D,E,F on B C , A C , A B BC,AC,AB respectively such that A D B C , B E A C , C F A B AD\perp BC,BE\perp AC,CF\perp AB . Without loss of generality, let D ( 0 , 0 ) , C ( 1 , 0 ) , B ( 1 , 0 ) D(0,0),C(1,0),B(-1,0) . Let point O O be the orthocentre and let the inradius be r r . This means that G ( 0 , 2 r ) G(0,2r) . Finally let A ( 0 , a ) A(0,a) .

Now, we get the equation of B A BA to be y = a x + a y=ax+a . This implies that the equation of C F CF is y = 1 a x + 1 a y=-\frac{1}{a}x+\frac{1}{a} . We know that G G lies on C F CF and has x x coordinate 0, so subbing in x = 0 x=0 , we get G = ( 0 , 1 a ) = ( 0 , 2 r ) a = 1 2 r G=(0,\frac{1}{a})=(0,2r)\Rightarrow a=\frac{1}{2r} .

We also know that the area of triangle is given by A D × B C ÷ 2 = a = 1 2 r AD\times BC \div 2=a=\frac{1}{2r} . Now, we shall find another way to obtain the area so we can get an equation and solve for r r . It is also evident that the area of the triangle is given by B C × r ÷ 2 + A B × r = r + r a 2 + 1 = r + 4 r 2 + 1 2 BC\times r\div2 +AB\times r=r+r\sqrt{a^2+1}=r+\frac{\sqrt{4r^2+1}}{2} . Equating, we have r + 4 r 2 + 1 2 = 1 2 r r 4 r 2 + 1 + 2 r 2 = 1 2 r 2 1 = r 4 r 2 1 4 r 4 4 r 2 + 1 = 4 r 4 + r 2 5 r 2 = 1 r = 1 5 a = 5 2 r+\frac{\sqrt{4r^2+1}}{2}=\frac{1}{2r}\\r\sqrt{4r^2+1}+2r^2=1\\2r^2-1=-r\sqrt{4r^2-1}\\4r^4-4r^2+1=4r^4+r^2\\5r^2=1\\r=\frac{1}{\sqrt{5}}\Rightarrow a=\frac{\sqrt{5}}{2} Since A C = a 2 + 1 AC=\sqrt{a^2+1} , we have A C = 3 2 AC=\frac{3}{2} and thus, A C B C = 3 2 2 = 3 4 \frac{AC}{BC}=\frac{\frac{3}{2}}{2}=\frac{3}{4} and the answer is 7.

Luna Biswas
Jun 25, 2016

this problem appeared in inmo 2016. so please see the solution either from the official site or from aops

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