Straight Lines And Pair Of Straight Lines

$\left|\frac{2 a b}{a^2+b^2}\right|$

$\left|\frac{a-b}{a+b}\right|$

$\left|\frac{a^2-b^2}{2 a b}\right|$

$\left|\frac{a^2-b^2}{a^2+b^2}\right|$