Question

sina+sinb=1/4,cosa+cosb=1/3,cos(a-b)=

Answer 1

(sina+sinb)^2=(sina)^2+(sinb)^2+2*sinasinb=1/16

(cosa+cosb)^2=(cosa)^2+(cosb)^2+2*cosacosb=1/9

以上两式相加,得1+1+2*（cosacosb+sinasinb）=25/144

cos(a-b)=cosacosb+sinasinb=-263/288