(tan5°-tan85°)(cos70°/1+cos20°)
问题描述：
(tan5°-tan85°)(cos70°/1+cos20°)
如题
答
cos20=sin70原式=(sin5/cos5-cos5/sin5)cos70/(1+sin70)=[(sin5)^2-(cos5)^2]/sin5/cos5*cos70/(1+sin70)=-2cos10/sin10*sin20/(1+sin70)=-2cos10/sin10*sin10*cos10/(1+sin70)=-2*(1+cos20)/(1+sin70)=-2