Let A B C be a triangle in which A B = A C . Suppose the orthocenter of the triangle lies on the incircle. If the value of B C A B can be expressed as b a , where a and b are coprime positive integers , entre your answer as a + b .
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Construct points D , E , F on B C , A C , A B respectively such that A D ⊥ B C , B E ⊥ A C , C F ⊥ A B . Without loss of generality, let D ( 0 , 0 ) , C ( 1 , 0 ) , B ( − 1 , 0 ) . Let point O be the orthocentre and let the inradius be r . This means that G ( 0 , 2 r ) . Finally let A ( 0 , a ) .
Now, we get the equation of B A to be y = a x + a . This implies that the equation of C F is y = − a 1 x + a 1 . We know that G lies on C F and has x coordinate 0, so subbing in x = 0 , we get G = ( 0 , a 1 ) = ( 0 , 2 r ) ⇒ a = 2 r 1 .
We also know that the area of triangle is given by A D × B C ÷ 2 = a = 2 r 1 . Now, we shall find another way to obtain the area so we can get an equation and solve for 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 + 2 4 r 2 + 1 . Equating, we have r + 2 4 r 2 + 1 = 2 r 1 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 = 5 1 ⇒ a = 2 5 Since A C = a 2 + 1 , we have A C = 2 3 and thus, B C A C = 2 2 3 = 4 3 and the answer is 7.
this problem appeared in inmo 2016. so please see the solution either from the official site or from aops
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