(x+y)^2=7^2x^2+2xy+y^2=49即25+2xy=492xy=24(x-y)^2=x^2-2xy+y^2=25-24=1x>y则x-y>0所以x-y=1
正负1
x-y=1
x+y=7∴(x+y)^2=49∴x^2+y^+2xy=49∵x^2+y^2=25∴xy=12,4xy=48∵x大于y，∴x-y＞0x-y=根号（(x+y)^2-4xy）=1
