∫x^3/（9+x^2）dx
=∫[x(x^2+9)-9x]/（9+x^2）dx
=1/2x^2-9∫x/（9+x^2）dx
=1/2x^2-4.5ln(9+x^2) +C
∫x^3/（9+x^2）dx
=1/2∫x^2/（9+x^2）dx^2
=1/2∫[1-9/（9+x^2）]dx^2
=1/2x^2-9ln(9+x^2)C
∫x^3dx/(9+x^2)
=(1/2)∫x^2dx^2/(9+x^2)
=(1/2)∫dx^2-(9/2)∫d(x^2+9)(9+x^2)
=(1/2)x^2-(9/2)ln(9+x^2)+C
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