1 / sin50° + √3 / cos50°=(cos50°+√3sin50°)/(sin50°cos50°)={2[(1/2)cos50°+(√3/2)sin50°]}/(sin50°cos50°)=2(cos60°cos50°+sin60°sin50°)/(sin50°cos50°)=4cos10°/(2sin50°cos50°)=4cos10...4cos10°/sin100°=4cos10°/cos10°这一步怎么来的？sin100°=sin(90°+10°)=cos10°诱导公式sin(90°+a)=cosa