.b=a.c 所以sinB·sinB=sinA·sinC
cos(A-C)+cosB=cos(A-C)-cos(A+C)
=cosA·cosC+sinA·sinC-(cosA·cosC+sinA·sinC)
=2sinA·sinC
=2sinB·sinB=3/2
所以sinB=更号3/2
B=60度或者120度
讨论:当B=120度时,cosB
.b=a.c 所以sinB·sinB=sinA·sinC
cos(A-C)+cosB=cos(A-C)-cos(A+C)
=cosA·cosC+sinA·sinC-(cosA·cosC+sinA·sinC)
=2sinA·sinC
=2sinB·sinB=3/2
所以sinB=更号3/2
B=60度或者120度
讨论:当B=120度时,cosB