x的平方-xy分之x的平方-xy 除以(x-y)乘xy分之x的平方-xy
=1×1/(x-y)×x(x-y)/xy
=1/yx的平方-xy分之x的平方+xy除以(x-y)乘xy分之x的平方-xy=x(x+y)/x(x-y)×1/(x-y)×x(x-y)/xy=(x+y)/y(x-y)=(x+y)/(xy-y²)