两式相减
a^2-b^2=9
相加a^2+2ab+b^2=21
(a+b)^2=21
(a+b)(a-b)=9
(a+b)^2(a-b)^2=81
(a-b)^2=81/(a+b)^2=81/21=27/7
a^2+b^2=[(a+b)^2+(a-b)^2]/2
=(21+27/7)/2
=60/7