Question

How to solve x^2+x-90=0 in completing the square method​

Answer 1

Rearrange the equation by writing

x² + x - 90 = 0

x² + x = 90

x² + x + 1/4 = 90 + 1/4

Recall that (a + b)² = a² + 2ab + b². Then

x² + 2 • x/2 + (1/2)² = 361/4

(x + 1/2)² = 361/4

x + 1/2 = ± √(361/4)

x = -1/2 ± 19/2

x = 9   or   x = -10