（3x/x-1-x/x+1)*x^2-1/2x,其中x=-3
=(3x^2+3x-x^2+1)/(x-1)(x+1)*(x+1)(x-1)/2x
=(2x^2+x+1)/2x
=x+1/2+1/2x
=-3+1/2-1/6
=-8/3