1/2（cos2a+cos2b）=cos（a+b）cos（a-b）证明：右边=(cosacosb-sinasinb)(cosacosb+sinasinb)=(cosacosb)^2-(sinasinb)^2=cos^2a(1-sin^2b)-(1-cos^2a)sin^2b=cos^2a-cos^2asin^2b-sin^2b+cos^2asin^2b=cos^2a-sin^...