cos2acos2b=(cos^2a-sin^2a)(cos^2b-sin^2b)
所以化简得：1/2(sin^2asin^2b+cos^2acos^2b+cos^2asin^2b+sin^2acos^2b)
=1/2(sin^2a+cos^2a)(sin^2b+cos^2b)
=1/2