Why do I get the wrong answer when factoring?

Question

2x^2 − 7x − 4

so I take A*C to get -8. Then find what adds to get -7, but multiples to get -8. I found that -1 and 8 do this... So I replace -7x with... -1 and 8

2x^2-1x+8x-4

Then I look, and I try and do Factor By Grouping...

x(2x-1)+4(2x-1)

I take out an X from 2x^2-1x, and am left with 2x-1.  Then I take out a 4 from 8x-4 and am left with 2x-1. Right?

So my final answer is

(2x-1)(x+4)

because you leave whats in the () and combine the two outside the ()... since it is a positive 4, it should be +4..

But when I ever I enter it into my online assignment, it says I am wrong. It says the answer should be..

(2x+1)(x-4)

I do not know why the signs are switched. Can anyone help?! I have made sure the problem is written correctly and it is. I don't know why my signs are wrong on every problem!

Ms. Worth · Answer

You are looking for the 2 factors of - 4 which add to - 7 AFTER YOU DOUBLE ONE OF THEM

( 2x      )( x      ) <-- this one will be doubled when you FOIL

The factors that do that are 
- 4 (doubled will become - 8)
+ 1

( - 4 ) * ( + 1 ) = - 4
2 ( - 4 ) + ( + 1 ) = - 7

Answer:
( 2x + 1 )( x - 4 )

ranjankar · Answer

2x^2 - 7x - 4

(2) ( -4) = -8

( -8) ( +1) = -8

2x^2 -8x + x - 4

2x ( x-4) + 1 ( x-4)

( 2x +1) ( x-4) ANSWER

Ryan · Answer

Because it's 2x^2-7x-4 not 2x^2+7x-4... It's just a simple mistake that you probably didn't notice at the time

Trending News

Why do I get the wrong answer when factoring?

3 Answers