Question

cos2x=2cosx
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Answer 1

cos2x=2cos^2x-1
2cos^2x-2cosx-1=0
cosx=t
2t^2-2t-1=0
t=1/2(1+-sqrt(1+2))
t=(1+-sqrt(3))/2
|t|<=1
t=(1-sqrt(3))/2
x=arccos(1-√3)/2+2пk