Question

已知sina=12/13,sin(A+B)=4/5,A,B均为锐角，求cosB/2

写出过程来。

Answer 1

cosA=5/13,
sin(A+B)=sinC=4/5,cosC=3/5,
cosB=-5/13*3/5+12/13*4/5=33/65,
cos(B/2)=((33/65+1)/2)^(1/2)=7/((65)^(1/2))