（1）已知a²+b²=8,a+b=4,求a-b的值2ab=(a+b)²-(a²+b²)=16-8=8;(a-b)²=8-8=0;所以a-b=0；（2）已知xy=3,x-y=7,求x²+y²、x+y的值x²+y²=(x-y)²+2xy=49+6=55;(x...