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 $\displaystyle A = a^2$ and $\displaystyle B = b^2$.
So we have $\displaystyle A^2 - 7AB - 18B^2$.
You should be able to factorise this by inspection...
$\displaystyle (A-9B)(A+2B)$
$\displaystyle (a^2 - 9b^2)(a^2 + 2b^2)$
Now you have a Difference of Two Squares as the first factor. Factorising that gives
$\displaystyle (a + 3b)(a - 3b)(a^2 + 2b^2)$.