2.已知x+y=3,xy=2,求x²+y²的值 3.已知x-y=5,x²+y²=15,求xy的值2.已知x+y=3,xy=2,求x²+y²的值3.已知x-y=5,x²+y²=15,求xy的值4.已知（a+b）²=17,ab=3.求（a-b）²的值

2.已知x+y=3,xy=2,求x²+y²的值 3.已知x-y=5,x²+y²=15,求xy的值2.已知x+y=3,xy=2,求x²+y²的值3.已知x-y=5,x²+y²=15,求xy的值4.已知（a+b）²=17,ab=3.求（a-b）²的值
2.已知x+y=3,xy=2,求x²+y²的值 3.已知x-y=5,x²+y²=15,求xy的值
2.已知x+y=3,xy=2,求x²+y²的值
3.已知x-y=5,x²+y²=15,求xy的值
4.已知（a+b）²=17,ab=3.求（a-b）²的值

2.已知x+y=3,xy=2,求x²+y²的值 3.已知x-y=5,x²+y²=15,求xy的值2.已知x+y=3,xy=2,求x²+y²的值3.已知x-y=5,x²+y²=15,求xy的值4.已知（a+b）²=17,ab=3.求（a-b）²的值
2) x平方+y平方
=(x+y)平方-2xy
=9-4=5
3) x-y=5
∴x平方-2xy+y平方=25
∵x平方+y平方=15
∴15-2xy=25
∴xy=-5
4)(a-b)平方
=a平方-2ab+b平方
=(a+b)平方-4ab
=17-12=5