不定积分 ：∫ x^3/√1+x^2 dx

令x = tanθ,dx = sec²θ dθ
∫ x³/√(1 + x²) dx
= ∫ tan³θ/|secθ| * (sec²θ dθ)
= ∫ sin³θ/cos⁴θ dθ
= ∫ tan²θ * secθtanθ dθ
= ∫ (sec²θ - 1) d(secθ)
= (1/3)sec³θ - secθ + C
= (1/3)[√(1 + x²)]³ - √(1 + x²) + C
= (1/3)(x² - 2)√(1 + x²) + C