Question

x2 + 7x - 18 Select one of the factors of the quadratic expression. A) (x+2) B) (x+6) C) (x-2) D) (x-6)

Answer 1

The factors of this expression would be (x+9)(x-2). You have to find two numbers that are factors of -18 that add up to 7. These numbers would be 9 and -2:
(9)x(-2)= -18
and 9 + (-2)= 7
Your answer is C) (x-2).

Answer 2

The right answer is C) (x - 2).

Explanation:

Given x² + 7x -18 , to find the correct factor you have to apply Baskhara´s equation

x = [-b ± √(b²- 4·a·c)]/2·a where a,b and c are the coeficietns of the quadratic equation, with a= 1, b= 7 and c= -18.

When you replace each term on the formula, you have

x= [-7 ± √(7² - 4·1·(-18)]/ 2·1 ⇒ x = [-7 ÷ √121]/2 ⇒ then you will find two solutions x1 and x2.

x1 = (-7 + 11)/2 ⇒ x1= 2

x2 = (-7 - 11)/2 ⇒x2=-9

Here you have the factors of the given equation.

So, x² + 7x -18 = (x-2) · (x +9).

Summarizing, the correct option is C) (x - 2)