a=b+4
a-b=4
(a-b)^2=16
a^2-2ab+b^2=16
a^2+b^2=18
(a+b)^2-2ab=18
(a+b)^2=20
则a+b=2根号5或负2根号5
因为 a-b=4
则a=2+根号5或2-根号5
b=根号5-2或-2-根号5