Question

Solve x2 = 12x – 15 by completing the square. Which is the solution set of the equation?

(negative 6 minus StartRoot 51 EndRoot comma negative 6 + StartRoot 51 EndRoot)
(negative 6 minus StartRoot 21 EndRoot comma negative 6 + StartRoot 21 EndRoot)
(6 minus StartRoot 51 EndRoot comma 6 + StartRoot 51 EndRoot)
(6 minus StartRoot 21 EndRoot comma 6 + StartRoot 21 EndRoot)

Answer 1

x^2 = 12x − 15
x^2 − 12x + 15 = 0
Complete the square: (x − 6)^2 − 36 + 15 = 0 → (x − 6)^2 = 21
x − 6 = ±√21 → x = 6 ± √21

So the solution set is (6 − √21, 6 + √21).