Question

Which shows the factored form of x2 – 12x – 45?

(x + 3)(x – 15)
(x – 3)(x – 15)
(x + 3)(x + 15)
(x – 3)(x + 15)

Answer 1

Hi!

(x+a)·(x+b) = x²+xa+xb+ab = x²-12x-45

x² = x²
xa+xb = (a+b)x = -12x ⇒ (a+b) = -12
ab = -45

When (a+b) = -12 and ab = -45?

+3-15 = -12 and (+3)(-15) = -45
-3-15 = -18 and (-3)(-15) = +45
+3+15 = +18 and (+3)(+15) = +45
-3+15 = +12 and (-3+15) = -45

Answer:

(x+3)(x-15)