Question

Marvin was completing the square, and his work is shown below. Identify the line where he made his mistake. f(x) = 2x2 − 8x + 5. Line 1: f(x) = 2(x^2 − 8x) + 5 Line 2: f(x) = 2(x^2 − 8x + 16) + 5 − 32 Line 3: f(x) = 2(x − 4)^2 − 27

Answer 1

2x^2 - 8x + 5....move the 5 to the other side
2x^2 - 8x = -5....now divide by 2
x^2 - 4x = -5/2
x^2 - 4x + 4 = -5/2 + 4
(x - 2)^2 = -5/2 + 8/2
(x - 2)^2 = 3/2
x - 2 = (+-)sqrt 3/2
x = 2 + sqrt 3/2 or x = 2 - sqrt 3/2

basically, he made his mistake on line 1