1987 AIME Problem 4

Attempt Problem 4 of the 1987 AIME below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1987 AIME solutions, or check the answer key.

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4.

Find the area of the region enclosed by the graph of x60+y=x4.|x-60|+|y|=|\frac{x}{4}|.

Answer: 480
Concepts:absolute valuecoordinate geometrykite
Difficulty rating: 1810
Small Hint:

Solve for y|y| and determine where its right-hand side is nonnegative

Big Hint:

The boundary is a kite whose vertices occur at the breakpoints of the absolute values

Solution:

We need y=x4x600.|y|=\frac{|x|}{4}-|x-60|\geq0. This forces 48x80.48\leq x\leq80. At x=48,x=48, x=60,x=60, x=80,x=80, the boundary values are respectively y=0,y=0, y=±15,y=\pm15, y=0.y=0. Hence the region is a kite with perpendicular diagonals 8048=3280-48=32 and 30,30, so its area is 12(32)(30)=480.\frac12(32)(30)=480.

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