Hello,

I can't figure out how I should factor this expression.

$\displaystyle x^4 - 18x^2 + 1$

I know the answer is $\displaystyle (x^2 - 4x - 1)(x^2 + 4x - 1)$

But I don't know how it gets to this point. I've tried substituting x squared with y to get this...

$\displaystyle y^2 - 18y + 1$

but this can't really be factored much more, so I don't know what I should be doing..