∵x-y+3的绝对值与2(x+y)的平方互为相反数
即│x-y+3│=-4(x+y)²==>│x-y+3│+4(x+y)²=0
∴x+y=0,x-y+3=0
故x的平方+2xy+y的平方=x²+2xy+y²
=(x+y)²
=0²
=0