1.已知x+y=4,x²-y²=20,则x-y的值为多少?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?