You are Here: Home >< Maths

# factorising cubic polynomials :( watch

1. hey, me again with a maths question

basically i watched this video on factorising cubics:

it essentially finds the first factor then uses long division.

i understand that method totally, but when i tried it on this question:

f(x) = 2x^3 + x^2 -8x - 4

i found that f(2) was a factor (the equation came to 0). in the video it says to use the negative version of it (so x-2).

however, when i used x-2 in long division it came out with the wrong answer, and in the answer book it uses x+2!

how come in this circumstance they don't use the negative version?

send help!!
2. (Original post by louisedrake)
send help!!
What you've done is entirely correct, it is a coincidence that in this case, both so that using or is correct.

In fact, your polynomial factorises as , so the book using the negative version, except with . But your method is just as correct.
3. (Original post by louisedrake)
hey, me again with a maths question
So, if you used then you should (after long division) get

If you used then you should (after long division) get .
4. (Original post by Zacken)
What you've done is entirely correct, it is a coincidence that in this case, both so that using or is correct.

In fact, your polynomial factorises as , so the book using the negative version, except with . But your method is just as correct.
thanks so much! x

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 28, 2016
Today on TSR

### He lied about his age

Thought he was 19... really he's 14

### University open days

Wed, 25 Jul '18
2. University of Buckingham
Wed, 25 Jul '18
3. Bournemouth University
Wed, 1 Aug '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams