Maths Olympiad Prep

Library / /154 of 520

Geometry Difficulty 5.3 AIME, harder Find the answer

11.8 Find the area of the figure defined on the coordinate plane by the inequality x2+y22(xy)x^{2}+y^{2} \leq 2(|x|-|y|).

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

Solution

Answer: 2π42 \pi-4.

!

Solution. It is obvious that the figure

is symmetric with respect to the coordinate axes and the origin (since the inequality does not change when the signs of x,yx, y are changed). Therefore, it is sufficient to consider the part of the figure in the first quadrant and multiply the area of this part by 4. For non-negative x,yx, y, the inequality can be written as (x1)2+(y+1)22(x-1)^{2}+(y+1)^{2} \leq 2. Thus, in the first quadrant, we have a part of a circle with radius 2\sqrt{2} and center at (1;1)(1; -1) (see the figure). This part of the circle is a segment with a central angle of 9090^{\circ} (this is the angle between the two radii drawn to the boundary points (0;0)(0; 0) and (2;0)(2; 0) of this segment). Therefore, the area of the segment is π(2)2/41\pi(\sqrt{2})^{2} / 4 - 1, and the area of the figure is

2π42 \pi-4.

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: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.