Maths Olympiad Prep

Library / /136 of 348

Geometry Difficulty 4.8 AIME Find the answer

Points D,E,FD, E, F lie on circle OO such that the line tangent to OO at DD intersects ray EF\overrightarrow{E F} at PP. Given that PD=4,PF=2P D=4, P F=2, and FPD=60\angle F P D=60^{\circ}, determine the area of circle OO.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

By the power of a point on PP, we get that 16=PD2=(PF)(PE)=2(PE)PE=816=P D^{2}=(P F)(P E)=2(P E) \Rightarrow P E=8. However, since PE=2PDP E=2 P D and FPD=60\angle F P D=60^{\circ}, we notice that PDEP D E is a 30609030-60-90 triangle, so DE=43D E=4 \sqrt{3} and we have EDDPE D \perp D P. It follows that DED E is a diameter of the circle, since tangents the tangent at DD must be perpendicular to the radius containing DD. Hence, the area of the circle is (12DE)2π=12π\left(\frac{1}{2} D E\right)^{2} \pi=12 \pi

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.