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2023 AMC 10B Problem 22

Problem 22 of 25HarderAlgebraProblem-Solving Techniques

How many distinct values of xx satisfy ⌊x⌋2−3x+2=0,\lfloor x \rfloor^2 - 3x + 2 = 0, where ⌊x⌋\lfloor x \rfloor denotes the largest integer less than or equal to x?x?

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Solution

Set n=⌊x⌋.n = \lfloor x \rfloor. Then n2−3x+2=0n^2 - 3x + 2 = 0 gives x=n2+23.x = \frac{n^2 + 2}{3}. For this to be consistent we need n≤n2+23<n+1.n \le \frac{n^2 + 2}{3} \lt n + 1. The left side, n2−3n+2≥0,n^2 - 3n + 2 \ge 0, holds for every integer n.n. The right side, n2−3n−1<0,n^2 - 3n - 1 \lt 0, holds only for n∈{0,1,2,3}.n \in \{0, 1, 2, 3\}. Those give x=23,1,2,113,x = \frac{2}{3}, 1, 2, \frac{11}{3}, so there are 44 distinct values. Therefore, the answer is B.
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Tagged: floor and ceiling functions · substitution · bounding to limit cases

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