Question

cos165°=______

cos165°=______．

Answer 1

cos165°=cos（180°-15°）=-cos15°=-cos（45°-30°）=-cos45°cos30°-sin45°sin30°=-

6 + 2

4

．
故答案为：-

6 + 2

4

Answer 2

cos165度=cos(180度－15度）=－cos15度=－cos(45度－30度)=－(cos45度cos30度十sin45度sin30度)=一√2/2×√3/2十√2/2×1/2=一√6+√2/4