三角函数求(2sin5°
求 (2sin5°-c25°)/sin25° 的值
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