x-x^2分之x^2-1×1-y分之xy-x=(x+1)(x-1)/x(1-x)*[x(y-1)]/(1-y)=(x+1)/(-x)*(-x)=x+1（xy-x^2）÷xy分之x-y=x(y-x)*xy/(x-y)=-x^2y x^2+x分之x-3÷（x^2-6x+9）=(x-3)/x(x+1)*1/(x-3)^2=1/[x(x+1)(x-3)]
