Maths Olympiad Prep

Library / /277 of 348

Geometry Difficulty 5.1 AIME, harder Find the answer

Let Ω\Omega be a circle of radius 8 centered at point OO, and let MM be a point on Ω\Omega. Let SS be the set of points PP such that PP is contained within Ω\Omega, or such that there exists some rectangle ABCDA B C D containing PP whose center is on Ω\Omega with AB=4,BC=5A B=4, B C=5, and BCOMB C \| O M. Find the area of SS.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We wish to consider the union of all rectangles ABCDA B C D with AB=4,BC=5A B=4, B C=5, and BCOMB C \| O M, with center XX on Ω\Omega. Consider translating rectangle ABCDA B C D along the radius XOX O to a rectangle ABCDA^{\prime} B^{\prime} C^{\prime} D^{\prime} now centered at OO. It is now clear that that every point inside ABCDA B C D is a translate of a point in ABCDA^{\prime} B^{\prime} C^{\prime} D^{\prime}, and furthermore, any rectangle ABCDA B C D translates along the appropriate radius to the same rectangle ABCDA^{\prime} B^{\prime} C^{\prime} D^{\prime}. We see that the boundary of this region can be constructed by constructing a quarter-circle at each vertex, then connecting these quarter-circles with tangents to form four rectangular regions. Now, splitting our region in to four quarter circles and five rectangles, we compute the desired area to be 414(8)2π+2(48)+2(58)+(45)=164+64π4 \cdot \frac{1}{4}(8)^{2} \pi+2(4 \cdot 8)+2(5 \cdot 8)+(4 \cdot 5)=164+64 \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.