Question

Use completing the square to solve (x – 12)(x + 4) = 9 for x. x = –1 or 15 x = 1 or 7

Answer 1

(x - 12)(x + 4) = 9
x² + 4x - 12x - 48 = 9
x² - 8x - 48 = 9    |add 48 to both sides
x² - 8x = 57
x² - 2x · 4 = 57    |add 4² to both sides
x² - 2x · 4 + 4² = 57 + 4²    |use (a - b)² = a² - 2ab + b²
(x - 4)² = 57 + 16
(x - 4)² = 73 ⇔ x - 4 = √73 or x - 4 = -√73    |add 4 to both sides

x = 4 + √73 or x = 4 - √73

Answer 2

The answer is D. $x= 4+/-\sqrt{73}$