# Thread:

1. ## Factoring Polynomials Help

So I have to do a review in my Precalculus textbook from Algebra 2 and their are these 3 questions that I just don't seem to understand. I have the answer in the back of the book but I want to know how to get to the answer.

Questions are:

1-8x^2-9x^4 - Answer is -(3x-1)(3x+1)(x^2-1)

(x+2)^2 - 5(x+2) - Answer is - (x+2)(x-3)

3(x^2+10x+25) - 4(x+5) - Answer is (x+5)(3x+11)

I Just don't undestand how they got to those answers.

2. Originally Posted by Kros
So I have to do a review in my Precalculus textbook from Algebra 2 and their are these 3 questions that I just don't seem to understand. I have the answer in the back of the book but I want to know how to get to the answer.

Questions are:

1-8x^2-9x^4 - Answer is -(3x-1)(3x+1)(x^2-1)

(x+2)^2 - 5(x+2) - Answer is - (x+2)(x-3)

3(x^2+10x+25) - 4(x+5) - Answer is (x+5)(3x+11)

I Just don't undestand how they got to those answers.
1) $1 -8x^2 - 9x^4 = -9x^4 - 8x + 1$

We need two numbers that multiply to become $-9$ and add to become $-8$. They are $-9$ and $1$.

$-9x^2 - 8x + 1 = -9x^4 - 9x^2 + 1x^2 + 1$

$= -9x^2(x^2 + 1) + 1(x^2 + 1)$

$= (x^2 + 1)(1 - 9x^2)$

$= (x^2 + 1)[1^2 - (3x)^2]$

$= (x^2 + 1)(1 - 3x)(1 + 3x)$.

Double check the answer you have been given.

2) $(x + 2)^2 - 5(x + 2)$

Take out a common factor of $x + 2$

$= (x + 2)(x + 2 - 5)$

$= (x + 2)(x - 3)$.

3) $3(x^2+10x+25) - 4(x+5) = 3(x + 5)^2 - 4(x + 5)$

$= (x + 5)[3(x + 5) - 4]$

$= (x + 5)(3x + 15 - 4)$

$= (x + 5)(3x + 11)$.
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