∫ 5x³/(x²+9) dx
=∫ 5x²/(x²+9) xdx
=5/2 ∫ x²/(x²+9) dx²
=5/2 ∫ (x²+9-9)/(x²+9) dx²
=5/2 ∫ [1-9/(x²+9)] dx²
=5/2 [ x² - ∫ 9/(x²+9) dx²]
=5/2 [ x² - 9ln(x²+9) ]