8. Given $\triangle A B C$ with two sides $B C=a, A C$ $=b$, and the medians $A D, B E$ on sides $B C, A C$ are perpendicular to each other, then the length of the third side $A B$ expressed in terms of $a, b$ is
8. $\frac{1}{5} \sqrt{5\left(a^{2}+b^{2}\right)}$