# Math Help - Polynomials

1. ## Polynomials

How to factorise (a^4) - (7a^2 x b^2)- 18b^4??
Can show me step by step??

Btw, the answer is (a+3b)(a-3b)(a^2+2b^2)

2. Originally Posted by maybeline9216
How to factorise (a^4) - (7a^2 x b^2)- 18b^4??
Can show me step by step??

Btw, the answer is (a+3b)(a-3b)(a^2+2b^2)
Let $A = a^2$ and $B = b^2$.

So we have $A^2 - 7AB - 18B^2$.

You should be able to factorise this by inspection...

$(A-9B)(A+2B)$

$(a^2 - 9b^2)(a^2 + 2b^2)$

Now you have a Difference of Two Squares as the first factor. Factorising that gives

$(a + 3b)(a - 3b)(a^2 + 2b^2)$.