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2020 AMC 12B Problem 8

Problem 8 of 25EasierAlgebraNumber Theory

How many ordered pairs of integers (x,y)(x, y) satisfy the equation x2020+y2=2y?x^{2020} + y^2 = 2y?

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Solution

Completing the square gives x2020+(y1)2=1.x^{2020} + (y - 1)^2 = 1. Both terms are nonnegative, so x20201,x^{2020} \le 1, forcing x{1,0,1}.x \in \{-1, 0, 1\}. If x=0,x = 0, then (y1)2=1,(y - 1)^2 = 1, giving y=0y = 0 or y=2.y = 2. If x=±1,x = \pm 1, then x2020=1,x^{2020} = 1, so (y1)2=0(y - 1)^2 = 0 and y=1.y = 1. The solutions are (0,0),(0,2),(1,1),(0, 0), (0, 2), (1, 1), and (1,1)(-1, 1) — four in all. Thus, the correct answer is D.

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Concepts: completing the square · Diophantine Equation · bounding to limit cases

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