(2x-y-3)^2={2x-(y+3)}^2
=4x^2-4x(y+2)+(y+3)^2
=4x^2+y^2-4xy-8x+6y+9谢了顺便帮我解｛x+y+1｝{x+y-1}｛x+y+1｝{x+y-1}=｛(x+y)+1｝{(x+y)-1}=(x+y)^2-1=x^2+y^2+2xy-1