Question

How do you factor `a^4-18a^2+81`?

Answer 1

`(a-3)^2(a+3)^2`

Explanation:

`a^4-18a^2+81`

Factorise: `(a^2-9)(a^2-9)`

`=(a^2-3^2)(a^2-3^2)`

`=(a+3)(a-3)(a+3)(a-3)`

`=(a-3)^2(a+3)^2`