凑微分法∫x/√(1-x^2)dx =-1/2∫d(1-x^2)/√(1-x^2) =-1/2∫[(1-x^2)^(-1/2)]d(1-x^2) =-1/2*2*(1-x^2)^(1/2)+C = -√(1-x^2)+C
