若cos(x-30°)=5/13,-60°小于x小于30°,则cosx=
问题描述：
若cos(x-30°)=5/13,-60°小于x小于30°,则cosx=
答
-60°
=-√[1-cos²(x-30°)]
=-√[1-(5/13)²]=-12/13
cosx
=cos[(x-30°)+30°]
=cos(x-30°)cos30°-sin(x-30°)sin30°
=5/13×√3/2-(-12/13)×1/2
=(5√3+12)/26