SAT Math Multiple Choice Question 77: Answer and Explanation

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Question: 77

2. If x2 + 12x = 64 and x > 0, what is the value of x ?

  • A. 2
  • B. 4
  • C. 8
  • D. 16

Correct Answer: B

Explanation:

B To solve the quadratic equation, first set the equation equal to 0. The equation becomes x2 + 12x - 64 = 0. Next, factor the equation to get (x + 16)(x - 4) = 0. Therefore, the two possible solutions for the quadratic equation are x + 16 = 0 and x - 4 = 0, so x = -16 or 4. Since the question states that x > 0, x = 4 is the only possible solution. Another way to approach this question is to plug in the answers. Start with (B), x = 4. Plug 4 into the equation to get 42 + 12(4) = 64. Solve the left side of the equation to get 16 + 48 = 64, or 64 = 64. Since this is a true statement, the correct answer is (B).

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