Question

Solve by completing the square:

x² − x − 20 = 0

Answer 1

Given:

x² − x − 20 = 0

(−4) + (5) = 1

(−4) × (5) = −20

(x + 4) (x − 5) = x² − x − 20

x² + x − 20 = 0

Product = −20

Sum = +1

Numbers are +5 and –4

x² + 5x − 4x − 20 = 0

⇒ (x + 5) (x − 4) = 0

x = −5 or x = 4