xy分之x^2-y^2除以(x-y)
=(x-y)(x+y)/xy÷(x-y)
=(x+y)/xy
=x/xy+y/xy
=1/y+1/xx+y分之y+y^2-x^2分之xyx+y分之y+y^2-x^2分之xy=y/(x+y)+xy/(y²-x²)=y(y-x)/(y²-x²)+xy(y²-x²)=(y²-xy+xy)/(y²-x²)=y²/(y²-x²)=1-(y²/x²)