# Factorizing sum of two powers.

Is it possible to factorize, I'm trying to prove it isn't prime.

$x^4 + 15^x$

If for what values of x will the above be prime, also any general method of determining if a really large number is prime?

Thanks.

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I'm not sure if it can be factorised, but note that $2^4 + 15^2 = 241$ is a prime. As others have pointed out, if $x$ is odd, then so are $x^4$ and $15^x$ and hence their sum is even. – JavaMan Dec 11 '12 at 8:58