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Geometry Difficulty 5.7 AIME, harder Prove it United States

Problem:

Circles ω1\omega_{1} and ω2\omega_{2} intersect at points AA and BB. Segment PQPQ is tangent to ω1\omega_{1} at PP and to ω2\omega_{2} at QQ, and AA is closer to PQPQ than BB. Point XX is on ω1\omega_{1} such that PXQBPX \parallel QB, and point YY is on ω2\omega_{2} such that QYPBQY \parallel PB. Given that APQ=30\angle APQ = 30^{\circ} and PQA=15\angle PQA = 15^{\circ}, find the ratio AX/AYAX / AY.

Solution

Figure 1

Let CC be the fourth vertex of parallelogram APCQAPCQ. The midpoint MM of PQ\overline{PQ} is the intersection of the diagonals of this parallelogram. Because MM has equal power with respect to the two circles ω1\omega_{1} and ω2\omega_{2}, it lies on AB\overleftrightarrow{AB}, the circles' radical axis. Therefore, CC lies on AB\overleftrightarrow{AB} as well.

Using a series of parallel lines and inscribed arcs, we have:

APC=APQ+CPQ=APQ+PQA=ABP+QBA=PBQ=XPB \angle APC = \angle APQ + \angle CPQ = \angle APQ + \angle PQA = \angle ABP + \angle QBA = \angle PBQ = \angle XPB

where the last equality follows from the fact that PXQBPX \parallel QB.

We also know that BXP=180PAB=CAP\angle BXP = 180^{\circ} - \angle PAB = \angle CAP, so triangles BXPBXP and CAPCAP are similar. By the spiral similarity theorem, triangles BPCBPC and XPAXPA are similar, too.

By analogous reasoning, triangles BQCBQC and YQAYQA are similar. Then we have:

AXAY=AX/BCAY/BC=AP/CPAQ/CQ=AP2AQ2 \frac{AX}{AY} = \frac{AX / BC}{AY / BC} = \frac{AP / CP}{AQ / CQ} = \frac{AP^{2}}{AQ^{2}}

where the last equality holds because APCQAPCQ is a parallelogram. Using the Law of Sines, the last expression equals sin215sin230=23\frac{\sin^{2} 15^{\circ}}{\sin^{2} 30^{\circ}} = 2 - \sqrt{3}.

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