a^2+b^2=(a-b)^2+2ab=2^2+2=6.
a^2+b^2=(a-b)^2+2ab=4+2=6
(a-b)^2=a^2-2ab+b^2a^2+b^2=(a-b)^2+2ab=4+2=6
a-b=几啊?
