6 6 b - a ------- 2 2 b - a
(b^6 - a^6)/(b^2 - a^2)
6 6 a - b ------- 2 2 a - b
/ 2 2 \ / 2 2 \ \a + b + a*b/*\a + b - a*b/
4 4 2 2 a + b + a *b