I know that $x^4-4 = (x^2)^{^2}-(2)^{^2}=(x^2-2)\cdot (x^2+2)$.
But, why isn't reducible over $\Bbb Q$?
However, the answer in my book says that it is not reducible. However, if I can separate in factors, I understand that it should be reducible. So I’m confused about the answer to my book.