Question

Solve x2 + 9x + 8 = 0 by completing the square. What are the solutions?

Answer 1

Answer: (-1,-8)

Explanation:

x^2 + 9x + 8 = 0
x^2 + 9x = -8

Add (9/2)^2 to both sides

x^2 + 9x + (9/2)^2 = -8 + (9/2)^2
(x + 9/2)^2 = -8 + 81/4
(x + 9/2)^2 = -32/4 + 81/4
(x + 9/2)^2 = 49/4

Square root both sides

Sqrt (x + 9/2)^2 = sqrt 49/4
x + 9/2 = plus or minus 7/2

Set x + 9/2 = 7/2:

x + 9/2 = 7/2
x = 7/2 - 9/2
x = -2/2 = -1

Set x + 9/2 = -7/2:
x + 9/2 = -7/2
x = -7/2 - 9/2
x = -16/2 = -8