Absolute Value Equation Pairs

CAT 2020 Slot 2 · QA · Hard · Algebra

In how many ways can a pair of integers (x,a)(x, a) be chosen such that x22x+a2=0x^2 - 2|x| + |a - 2| = 0?

  1. A.

    7

  2. B.

    6

  3. C.

    4

  4. D.

    5

Answer

A

Explanation

Rewrite as a2=2xx2=2xx2=x(2x)|a - 2| = 2|x| - x^2 = 2|x| - |x|^2 = |x|(2 - |x|). Since a20|a - 2| \ge 0, we must have x(2x)0|x|(2 - |x|) \ge 0. Since x0|x| \ge 0, we need 2x0    x22 - |x| \ge 0 \implies |x| \le 2. Since xx is an integer, x|x| can be 0,1,20, 1, 2.

Case 1: x=0    x=0|x| = 0 \implies x = 0. a2=0    a=2|a - 2| = 0 \implies a = 2. Pair: (0,2)(0, 2) — 1 solution.

Case 2: x=1    x=1|x| = 1 \implies x = 1 or x=1x = -1. a2=1(21)=1    a2=1|a - 2| = 1(2 - 1) = 1 \implies a - 2 = 1 or 1    a=3-1 \implies a = 3 or a=1a = 1. Pairs: (1,3),(1,1),(1,3),(1,1)(1, 3), (1, 1), (-1, 3), (-1, 1) — 4 solutions.

Case 3: x=2    x=2|x| = 2 \implies x = 2 or x=2x = -2. a2=2(22)=0    a=2|a - 2| = 2(2 - 2) = 0 \implies a = 2. Pairs: (2,2),(2,2)(2, 2), (-2, 2) — 2 solutions.

Total pairs =1+4+2=7= 1 + 4 + 2 = 7.

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