i am given a question to show that $\displaystyle x^4-a^4= (x+a)(x^2+a^2)$

hence factorise $\displaystyle x^4-4= $ and find the roots of the equation..

i have used the binomial expansion theorem to prove the first part of the question but i am having difficulty in factorizing the second equation.... anyone can show me how this is done..

i write $\displaystyle x^4-4$ as $\displaystyle x^4-2^2$ but i still cant get it in a form to factorize it.