Помогите срочно надо

Решите задачу:

$|\bar{a}-\bar{b}|^2 = |\bar{a}|^2+|\bar{b}|^2-2|\bar{a}||\bar{b}|\cos(\bar{a};\bar{b}), \\ |\bar{a}+\bar{b}|^2 = |\bar{a}|^2+|\bar{b}|^2+2|\bar{a}||\bar{b}|\cos(\bar{a};\bar{b}), \\ \cos(\bar{a};\bar{b}) = \frac{|\bar{a}+\bar{b}|^2-|\bar{a}|^2-|\bar{b}|^2}{2|\bar{a}||\bar{b}|}, \\ |\bar{a}-\bar{b}|^2 = |\bar{a}|^2+|\bar{b}|^2-2|\bar{a}||\bar{b}|\frac{|\bar{a}+\bar{b}|^2-|\bar{a}|^2-|\bar{b}|^2}{2|\bar{a}||\bar{b}|} = 2|\bar{a}|^2+2|\bar{b}|^2-|\bar{a}+\bar{b}|^2, \\$
$|\bar{a}-\bar{b}|^2 = 2\cdot13^2+2\cdot19^2-20^2 = 660, |\bar{a}-\bar{b}| = 2\sqrt{165}.$