(with literal expressions)

Let $a$ and $b$ be positive real numbers. Factorize the following expressions:

$$
a^{4}-b^{4} \quad a+b-2 \sqrt{a b}
$$
$-a^{4}-b^{4}=\left(a^{2}-b^{2}\right)\left(a^{2}+b^{2}\right)=(a-b)(a+b)\left(a^{2}+b^{2}\right)$

$-a+b-2 \sqrt{a b}=(\sqrt{a}-\sqrt{b})^{2}$