【正文】
00800c os260s in 440c os280s in21 ?? ( 2))2c os ()23s i n()27c os ()2s i n()23s i n()s i n()3t a n(?????????????????????? 例 已知 ? ? 3175cos ???? ,且 ?? 90180 ???? ? ,求 ? ????15cos . 【課堂練習】 求證: ??? s in23c os ???????? ?, ??? c os23s in ???????? ?. 化簡: 0200020s in170c