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What are the solutions of the quadratic equation $4 x^{2}-8 x-12=0$ ?

SAT · Math · previous-year question

  1. A.$x=-1$ and $x=-3$
  2. B.$x=-1$ and $x=3$correct
  3. C.$x=1$ and $x=-3$
  4. D.$x=1$ and $x=3$

Answer

B. $x=-1$ and $x=3$

Explanation

Choice $\mathbf{B}$ is correct. Dividing both sides of the quadratic equation $4 x^{2}-8 x-12=0$ by 4 yields $x^{2}$ $-2 x-3=0$. The equation $x^{2}-2 x-3=0$ can be factored as $(x+1)(x-3)=0$. This equation is true when $x+1=0$ or $x-3=0$. Solving for $x$ gives the solutions to the original quadratic equation: $x=-1$ and $x=3$.Choices $A$ and $C$ are incorrect because -3 is not a solution of $4 x^{2}-8 x-12=0: 4(-3)^{2}-8(-3)-$ $12=36+24-12 \neq 0$. Choice $D$ is incorrect because 1 is not a solution of $4 x^{2}-8 x-12=0: 4(1)^{2}$ $-8(1)-12=4-8-12 \neq 0$

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