Passel of Techniques -- 2

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, find the ratio A B B C \dfrac{ AB }{ BC } .

If this ratio can be expressed as p q \frac pq , where p p and q q are coprime positive integers, submit your answer as p + q p+ q .


The answer is 7.

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1 solution

Ajit Athle
Dec 21, 2016

Let G be the midpoint of DC. Join G to I, the in-centre and extend it to meet AB in J. We note that GI//CH and thus meets AB at a right angle. Moreover, J is thus the point of tangency of the in-circle with side AB. We further claim that triangles BGJ & ABD are similar and therefore, AB/BD = BG/BJ =BG/BD. Thus, AB=BG=BC/2+BC/4=(3/4)BC which yields, AB/BC = 3/4 or p+q =7..

Nice method!

Dan Ley - 4 years, 5 months ago

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