5、in???2cossin???2???2???2·倍角公式:sin2??2sin?cos?cos2??2cos??1?1?2sin??cos??sin?cot2??tan2??cot??12cot?2tan?1?tan?222222sin3??3sin??4sin?cos3??4cos??3cos?tan3??3tan??tan?1?3tan?2333·半角公式:sintan?2?????cos?21?cos?1?cos?asinA 1?cos?sin?bsinB? cos cot?2??1?