若cos (x+30°)+sin x =4/5,则

若cos (x+30°)+sin x =4/5,则cos (2x-60°)等于?

3个回答

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