(a+b)^2=7得a^2+2ab+b^2=7----------1
(a-b)^2=4得a^2-2ab+b^2=4----------2
1 2两式相加得2（a^2+b^2)=11,所以a方+b方=11/2
1 2两式相减得4ab=3,所以ab=3/4