题目

求（1+x^1/3）^2 dx 的不定积分

提问时间：2020-10-08

答案

∫dx/(x²+x+1)^(3/2)
= ∫dx/[(x+1/2)²+3/4]^(3/2)
令x+1/2=√3/2*tanψ =>dx=√3/2*sec²ψ dψ
sinψ=(x+1/2)/√(x²+x+1),cosψ=(√3/2)/√(x²+x+1)
(x²+x+1)^(3/2)=(3/4*tan²ψ)^(3/2)=(3/4)^(3/2)*(sec²ψ)^(3/2)=(3√3/8)sec³ψ
=>∫(√3/2*sec²ψ)/(3√3/8 * sec³ψ) dψ
=>(4/3)∫cosψ dψ
=>(4/3)sinψ + C
=>(4/3) * (x+1/2)√(x²+x+1) + C
=>(2/3) * (2x+1)/√(x²+x+1) + C