The answer is -1 or 9
The quadratic equation is ax² + bx + c = 0. But, by completing the square we turn it into: a(x + d)² + e = 0, where:
d = b/2a
e = c - b²/4a
Our quadratic equation is x² - 8x = 9, which is after rearrangement:
x² - 8x - 9 = 0
So, a = 1, b = -8, c = -9
Let's first calculate d and e:
d = b/2a = -8/(2 * 1) = -8/2 = -4
e = c - b²/4a = -9 - (-8)²/(4 * 1) = -9 - 64/4 = -9 - 16 = -25
By completing the square we have:
a(x + d)² + e = 0
1(x + (-4))² + (-25) = 0
(x - 4)² - 25 = 0
(x - 4)² = 25
⇒ x - 4 = √25
Since √25 can be either -5 or +5 , then:
x - 4 = -5 or x - 4 = 5
x = -5 + 4 or x = 5 + 4
x = -1 or x = 9
