（-x）³·x^2n-1+x^2n·（-x）²

（-x）³·x^2n-1+x^2n·（-x）²=-x^3*x^2n-1+x^2n*x^2=-x^3+2n-1+x^2n+2=-x^2n+2+x^2n+2=0