解由余弦定理
c²=a²+b²-2abcosC
=（a+b）²-2ab-2abcos60°
=（a+b）²-3ab
即（√7）²=（5）²-3ab
即3ab=18
即ab=6
即SΔABC=1/2absinC
=1/2*6*√3/2
=3√3/2