原式=[(x+y-x+y)/(x-y)(x+y)]*[(x-y)(x-y)/2y] =(x-y)/(x+y) 我这用手机打不出根号所以根号我就打中文了 当x=1+根号2,y=1-根号2时,原式=(1+根号2-1+根号2)/(1+根号2+1-根号2) =2倍根号2/2 =根号2