A=B=C=60最大3/2根号3
证明：
sinA+sinB+sinc
=2sin((A+B)/2)cos((A-B)/2)+sinC
>=2sin((A+B)/2)+sinC
=2sin(90-C/2)+sinC
=2cos(C/2)+sinC
>=3sin60
=3/2根号3
当且仅当A=B=C=60取等号