若cos (x+30°)+sin x =4/5,则cos (2x-60°)等于?
问题描述：
若cos (x+30°)+sin x =4/5,则cos (2x-60°)等于?
答
解,cosxcos30°-sinxsin30°+sinx=4/5
√3/2cosx+1/2sinx=4/5
cos(x-30°)=4/5
cos(x-60°)=cos2(x-30°)=2cos^2(x-60°)-1=2*(4/5)^2=7/25解，cosxcos30°-sinxsin30°+sinx=4/5 √3/2cosx+1/2sinx=4/5 cos(x-30°)=4/5cos(x-60°)=cos2(x-30°)=2cos^2(x-60°)-1=2*(4/5)^2=7/25 请采纳。