若cos (x+30°)+sin x =4/5,则cos (2x-60°)等于?
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若cos(x+30°)+sinx=4/5,则cos(2x-60°)等于?若cos(x+30°)+sinx=4/5,则cos(2x-60°)等于?若cos(x+30°)+sinx=4/5,则cos(2x
若cos (x+30°)+sin x =4/5,则cos (2x-60°)等于?
若cos (x+30°)+sin x =4/5,则cos (2x-60°)等于?
若cos (x+30°)+sin x =4/5,则cos (2x-60°)等于?
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
这是个深奥的问题……
7/25
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