原式=∫((x+3)x^2-3x^2)/(x+3)dx=∫( x^2-(3(x+3)x-9x)/(x+3))dx
=∫(x^2-3x+9x/(x+3))dx=∫(x^2-3x+9-27/(x+3))dx=(x^3)/3-3(x^2)/2+9x-27ln|x+3|