SAT Math · Nonlinear equations in one variable · medium

What is the solution to √(2x+3) = x, after rejecting any extraneous candidate?

What is the solution to 2x+3=x\sqrt{2x+3} = x, after rejecting any extraneous candidate?

Answer and explanation

Answer: B: 3

Square both sides of 2x+3=x\sqrt{2x+3} = x to get 2x+3=x22x + 3 = x^2. Rearrange: x2−2x−3=0x^2 - 2x - 3 = 0, which factors as (x−3)(x+1)=0(x-3)(x+1) = 0, so x=3x = 3 or x=−1x = -1. Squaring can introduce extraneous solutions, so check both. For x=3x = 3: 9=3\sqrt{9} = 3, which is true. For x=−1x = -1: 1=1\sqrt{1} = 1, but the right side is −1-1, so it fails. The only solution is 3, choice B. Choice A (−1-1) is the extraneous root, choice C includes both without checking, and choice D (9) is the value under the radical, not xx.

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