Maths Olympiad Prep

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Geometry Difficulty 5.0 AIME, harder Prove it United States

Problem:

OKRAO K R A is a trapezoid with OKO K parallel to RAR A. If OK=12O K=12 and RAR A is a positive integer, how many integer values can be taken on by the length of the segment in the trapezoid, parallel to OKO K, through the intersection of the diagonals?

Solution

Solution:

Let RA=xR A = x. If the diagonals intersect at XX, and the segment is PQP Q with PP on KRK R, then PKXRKA\triangle P K X \sim \triangle R K A and OKXRAX\triangle O K X \sim \triangle R A X (by equal angles), giving RA/PX=AK/XK=1+AX/XK=1+AR/OK=(x+12)/12R A / P X = A K / X K = 1 + A X / X K = 1 + A R / O K = (x+12)/12, so PX=12x/(12+x)P X = 12x/(12+x). Similarly XQ=12x/(12+x)X Q = 12x/(12+x) also, so PQ=24x/(12+x)=2428812+xP Q = 24x/(12+x) = 24 - \frac{288}{12+x}. This has to be an integer. 288=2532288 = 2^{5} 3^{2}, so it has (5+1)(3+1)=18(5+1)(3+1) = 18 divisors. 12+x12 + x must be one of these. We also exclude the 8 divisors that don't exceed 12, so our final answer is 10.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.