∫ x^2 /(x^2+4) dx
=∫ 1 - 4/(x^2+4) dx
=∫ 1 - 1/ [(x/2)^2+1] dx
=x - ∫2/ [(x/2)^2+1] d(x/2)
=x -2 arctan(x/2) +C,C为常数