问题
选择题
在△ABC中，a，b，c分别为内角A、B、C所对的边，那么
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答案
由正弦定理知
=a sinA
=b sinB c sinC
代入得
-b•cosC-a bcosA-c
=sinC sinA
-sinBcosC-sinA sinBcosA-sinC sinC sinA
=
-sinBcosC-sinBcosc-cosBsinC sinBcosA-sinAcosB-cosAsinB sinC sinA
=
-cosBsinC sinAcosB sinC sinA
=
-sinC sinA
=0sinC sinA
故选B．