Skip to main content

2023 AMC 10B Problem 13

Problem 13 of 25IntermediateAlgebraGeometry

What is the area of the region in the coordinate plane defined by the inequality x1+y11?\bigl||x| - 1\bigr| + \bigl||y| - 1\bigr| \le 1?

Answer choices

Show solution

Solution

Substitute u=x,u = |x|, v=y.v = |y|. Then u1+v11|u - 1| + |v - 1| \le 1 is a diamond centered at (1,1)(1, 1) with diagonals of length 2,2, so it has area 2,2, and it sits entirely in u,v0.u, v \ge 0. The map (x,y)(x,y)(x, y) \mapsto (|x|, |y|) is four-to-one over u,v>0,u, v \gt 0, so the full region has area 4×2=8.4 \times 2 = 8. Thus, B is the correct answer.

More practice

Concepts: absolute value · coordinate geometry · area · symmetry

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.