# Thread: Factorising another polynomial over C

1. ## Factorising another polynomial over C

Hi there

I have a start on this polynomial:
$f(z) = x^4 - 2z^2 - 16z - 15$
If I let f(z) = f(-1) I will find that the expression equals zero and so (z+1) is a factor.
But where do I go from here?

The answer in the back of the book is (z+1)(z+1+2i)(z+1-2i)(z-3)

cheers
David.

2. I assume the function is actually $\displaystyle f(z) = z^4 - 2z^2 - 16z - 15$.

Since you've found a factor, long divide to get the cubic factor. See if you can factorise that cubic.