{(x+y)²-(x-y)²}÷(-4xy)=[x^2+2xy+y^2-(x^2-2xy+y^2]/(-4xy)=4xy/(-4xy)=-1
(3xy-y)÷y=3xy/y-y/y=3x-1还有（x+y)²÷（x+y）(x+y)^2/(x+y)=x+y告诉我（x+y)²÷（x+y）怎么直接得x+y的(x+y)^2÷(x+y)=(x+y)^(2-1)=x+y