绝对值和平方都大于等于0相加等于0则都等于0所以x-a=0,y+b=0x=a,y=-b[(a²+ay-bx+b²）/（x+y)]/[(a²+ax+by-b²)/(a+b)]=[(a²-2ab+b²）/（a-b)]/[(a²+a²-b²-b²)/(a+b)]=[(a-b)²/（a-b)]/[2(a+b)(a-b)/(a+b)]=(a-b)/2(a-b)=1/2
