cos^4 a-sin^4 a=?sin15°-cos15°=?
问题描述：
cos^4 a-sin^4 a=?sin15°-cos15°=?
答
cos^4 a-sin^4 a=?
=(cosa^2+sina^2)(cosa^2-sina^2)
=cos2a
sin15`-cos15°
=(sin15`-cos15°)/(sin15`^2+cos15°^2)
=(tan15`-1)/(1+tan15^2)
=-tan30=-根号3/3