∫x³/（9+x²）dx= ∫(x³+9x-9x)/（9+x²）dx=∫xdx-9∫x/(9+x²)dx=x²/2-9/2∫1/(9+x²)d(9+x²)=x²/2-9/2ln(9+x²)+c
解：=0.5 ∫x²/（9+x²）dx²=0.5（ ∫1-（9/（9+x²）)dx²=0.5 x²- ∫9/（9+x²)dx²=0.5 x²-9ln(9+ x²)+c
