S=1/2absinC
3根号3/2=1/2ab*根号3/2
ab=6
余弦定理得:c^2=a^2+b^2-2abcosC
4/49=(a+b)^2-2ab-2ab*1/2
4/49=(a+b)^2-12-6
(a+b)^2=886/49
a+b=根号886/7