分类：教育/科学-学习帮助
问题：三角函数求(2sin5°
 求 (2sin5°-c25°)/sin25° 的值
回答：cos25°=cos(30°-5°)=cos30°cos5°+sin30°sin5°=
(√3/2)cos5°+(1/2)sin5°
sin25°=sin(30°-5°)=sin30°cos5°-cos30°sin5°=
(1/2)cos5°-(√3/2)sin5°

∴(2sin5°-cos25°)/sin25°=
[2sin5°-(√3/2)cos5°-(1/2)sin5°]/sin25°=
[(3/2)sin5°-(√3/2)cos5°]/[(1/2)cos5°-(√3/2)sin5°]=
(√3/2)[√3sin5°-cos5°]/(1/2)[cos5°-√3sin5°]=
(√3/2)[√3sin5°-cos5°]/(-1/2)[-cos5°+√3sin5°]=
(√3/2)/(-1/2)=
-√3