Question

If x - 3 is a factor of $x^2$ - ax - 15, then a =

• A. -2
• B. 5
• C. -5
• D. 3

Answer 1

The correct option is A

-2

x - 3 is a factor of f(x) = $x^2$ - ax - 15

Let x - 3 = 0, then x = 3

$\therefore$ f(3) $=(3{)}^2$ -a(3) - 15

= 9 - 3a - 15 = - 6 - 3a

$\therefore$ x -3 is a factor

$\therefore$ Remainder = 0 $\therefore$ -6-3a=0$\Rightarrow$ 3a =-6

$\therefore$ a = $\frac{-6}{3}=-2$