# Factoring Question

• Apr 23rd 2010, 03:31 PM
cantstop01
Factoring Question
I am asked to factor the equation

$4x^2 - 4x - 15 = 0$

To my knowledge, your supposed to find pairs of factors that multiply to give -15 and add to give -4, correct?

I can't find a pair that gives both those answers.

I'm stuck.
• Apr 23rd 2010, 03:38 PM
skeeter
Quote:

Originally Posted by cantstop01
I am asked to factor the equation

$4x^2 - 4x - 15 = 0$

To my knowledge, your supposed to find pairs of factors that multiply to give -15 and add to give -4, correct?

I can't find a pair that gives both those answers.

I'm stuck.

$4x^2 - 4x - 15 = (2x-5)(2x+3)$
• Apr 23rd 2010, 03:55 PM
cantstop01
Thanks, looks like the answer is $x = 5/2$ and $x = -3/2$

I would like to ask one more thing.

what is the reasoning behind the factors not actually adding up to -4, yet still being the correct answer?
• Apr 23rd 2010, 04:12 PM
skeeter
Quote:

Originally Posted by cantstop01
Thanks, looks like the answer is $x = 5/2$ and $x = -3/2$

I would like to ask one more thing.

what is the reasoning behind the factors not actually adding up to -4, yet still being the correct answer?

that "technique" only works if the coefficient of $x^2$ is 1