>>209
x^2f(a)-a^2f(x) = (x^2 -a^2) f(a) - a^2 (f(x) - f(a))

{x^2f(a)-a^2f(x)}/(x-a) = (x+a) f(a) - a^2 { ( f(x) - f(a))/(x-a) }
→ 2a f(a) - a^2 f'(a)