AoPSWiki
Trying to get to the USAMO in 2010? Our AIME Problem Series can help you get there! Click here to enroll today!

1995 AIME Problems/Problem 9

From AoPSWiki

Problem

Triangle ABC is isosceles, with AB=AC and altitude AM=11. Suppose that there is a point D on \overline{AM} with AD=10 and \angle BDC=3\angle BAC. Then the perimeter of \triangle ABC may be written in the form a+\sqrt{b}, where a and b are integers. Find a+b.

Solution

Let x=\angle CAM, so 3x=\angle CDM. Then, \frac{\tan 3x}{\tan x}=\frac{CM/1}{CM/11}=11. Expanding \tan 3x using the angle sum identity gives \tan 3x=\tan(2x+x)=\frac{3\tan x-\tan^3x}{1-3\tan^2x}. Thus, \frac{3-\tan^2x}{1-3\tan^2x}=11. Solving, we get \tan x= \frac 12. Hence, CM=\frac{11}2 and AC= \frac{11\sqrt{5}}2 by the Pythagorean Theorem. The total perimeter is 2(AC + CM) = \sqrt{605}+11. The answer is thus a+b=\boxed{616}.

See also

1995 AIME (ProblemsResources)
Preceded by
Problem 8
Followed by
Problem 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Try our innovative online adaptive learning system, Alcumus.
Over 1100 problems and 60+ video lessons. FREE!
© Copyright 2008 AoPS Incorporated. All Rights Reserved. • FoundationPrivacyContact Us