7
advanced-math

Solve: x2+4x=1x^2 + 4x = 1

A

2±5-2 \pm \sqrt{5}

B

2±52 \pm \sqrt{5}

C

2±1-2 \pm 1

D

4±5-4 \pm \sqrt{5}

Correct Answer: A

Choice A is the correct answer. First, rewrite in standard form: x2+4x1=0x^2 + 4x - 1 = 0.

  1. Identify: a=1,b=4,c=1a=1, b=4, c=-1.
  2. Discriminant: 424(1)(1)=16+4=204^2 - 4(1)(-1) = 16 + 4 = 20.
  3. Formula: x=4±202x = \frac{-4 \pm \sqrt{20}}{2}.
  4. Simplify: 20=4×5=25\sqrt{20} = \sqrt{4 \times 5} = 2\sqrt{5}.
  5. x=4±252=2±5x = \frac{-4 \pm 2\sqrt{5}}{2} = -2 \pm \sqrt{5}.

Choice B is incorrect because of a sign error on the linear term. Choice C is incorrect because it simplifies 20\sqrt{20} incorrectly. Choice D is incorrect because it forgets to divide by 2.