若M=y/(x^2-xy),N=x/(y^2-xy),P=2/(x-y),那么：
M-N-P
=y/(x^2-xy) - x/(y^2-xy) - 2/(x-y)
=y/[x(x-y)] + x/[y(x-y)] - 2/(x-y)
=(y²+x²-2xy)/[xy(x-y)]
=(x-y)²/[xy(x-y)]
=(x-y)/(xy)
或可 =(1/y) - (1/x)