∫dx/(x³√(1+x²))=-1/2∫d(1/x²)/√(1+x²)=-1/2∫dt)/√(1+1/t²) 设1/x²=t=-1/2∫tdt/√(1+t²)=-1/4∫dt²/√(1+t²)=-1/4∫d(1+t²)/√(1+t²)=-√(1+t²)/2=-√(1+1/x^4)/2
