由余弦定理BC²=AB²+AC²-2AB*AC*cosA
=12²+6²-2*12*6*(-1/2)
=252
所以BC=6√7
由正弦定理AB/sinC=AC/sinB=BC/sinA=6√7/(√3/2)=4√21
所以sinC=AB/(4√21)=√21/7
sinB=BC/(4√21)=√21/14
故sinB+sinC=(3/14)√21
由余弦定理BC²=AB²+AC²-2AB*AC*cosA
=12²+6²-2*12*6*(-1/2)
=252
所以BC=6√7
由正弦定理AB/sinC=AC/sinB=BC/sinA=6√7/(√3/2)=4√21
所以sinC=AB/(4√21)=√21/7
sinB=BC/(4√21)=√21/14
故sinB+sinC=(3/14)√21