[（x+y）×（x-y）-（x-y)²+2y(x-y）]÷（-2y）=[x²-y²-(x²-2xy+y²)+2xy-2y²]÷(-2y)=(4xy-4y²)÷(-2y)=2y-2x
=(x2-y2-x2+2xy-y2+2xy-2y2)÷(-2y)=(-4y2+4xy)÷(-2y)=2y-2x
tgtgtrt
