原式=-(x-3)²/(x+3)(x-3)×x(x+3)/2(x-3)=-(x-3)/(x+3)×x(x+3)/2(x-3)=-x/2
x²-6x+9/9-x²除以2x-6/x²+3x=(x-3)²/(3-x)(3+x)×[x(x+3)/2(x-3)]=-x/2
