(1)由题可得
sin(a+b)=1213
cosb=4/5 所以cos(a+2b)=cos(a+b+b)=cos(a+b)cosb-sin(a+b)sinb=-56/65
sina=sin(a+b-b)=sin(a+b)cosb-sinbcos(a+b)=63/65
望采纳
(1)由题可得
sin(a+b)=1213
cosb=4/5 所以cos(a+2b)=cos(a+b+b)=cos(a+b)cosb-sin(a+b)sinb=-56/65
sina=sin(a+b-b)=sin(a+b)cosb-sinbcos(a+b)=63/65
望采纳