=[1/(x-3)-(x+1)/(x+1)(x-1)]×(x-3)=1-(x-3)/(x-1)=2/(x-1)
(x-3分之1-x的平方-1分之x+1)乘以(x-3) 因为1分之x=x，所以=(x-1/3-x²-x+1)(x-3)=(2/3-x²)(x-3)=2x/3-x³-2+3x²=-x³+3x²+2x/3-2
