Maths Olympiad Prep

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Geometry Difficulty 5.5 AIME, harder Prove it United States

Problem:

Suppose ABCDABCD is a rectangle whose diagonals meet at EE. The perimeter of triangle ABEABE is 10π10\pi and the perimeter of triangle ADEADE is nn. Compute the number of possible integer values of nn.

Solution

Solution:

For each triangle T\mathcal{T}, we let p(T)p(\mathcal{T}) denote the perimeter of T\mathcal{T}.

First, we claim that 12p(ABE)<p(ADE)<2p(ABE)\frac{1}{2} p(\triangle ABE) < p(\triangle ADE) < 2 p(\triangle ABE). To see why, observe that
p(ADE)=EA+ED+AD<2(EA+ED)=2(EA+EB)<2p(ABE) p(\triangle ADE) = EA + ED + AD < 2(EA + ED) = 2(EA + EB) < 2 p(\triangle ABE)
Similarly, one can show that p(ABE)<2p(ADE)p(\triangle ABE) < 2 p(\triangle ADE), proving the desired inequality.

This inequality limits the possibility of nn to only those in (5π,20π)(15.7,62.9)(5\pi, 20\pi) \subset (15.7, 62.9), so nn could only range from 16,17,18,,6216, 17, 18, \ldots, 62, giving 4747 values. These values are all achievable because

- when ADAD approaches zero, we have p(ADE)2EAp(\triangle ADE) \rightarrow 2EA and p(ABE)4EAp(\triangle ABE) \rightarrow 4EA, implying that p(ADE)12p(ABE)=5πp(\triangle ADE) \rightarrow \frac{1}{2} p(\triangle ABE) = 5\pi;
- similarly, when ABAB approaches zero, we have p(ADE)2p(ABE)=20πp(\triangle ADE) \rightarrow 2 p(\triangle ABE) = 20\pi; and
- by continuously rotating segments ACAC and BDBD about EE, we have that p(ADE)p(\triangle ADE) can reach any value between (5π,20π)(5\pi, 20\pi).

Hence, the answer is 4747.

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