令y=x2/(1+x2)则：x^2=y+yx^2 (1-y)x^2=y x^2=y/(1-y) x=[y/(1-y)]^(1/2)最后改写为函数形式
x-arctanx+C解：∫x²/(1+x²)dx=∫(1+x²-1)/(1+x²)dx=∫[1-1/(1+x²)]dx=x-arctanx+C
