Question

(3theta) =-1
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Answer 1

Hello :
cos(3θ) = -1
cos(3θ) = cos(π)
general solution in : R  is 
3θ = (2k+1)π   k in  Z 
θ = (2k+1)π/3
if  θ in   [0 , 2π]   the solutions are :
k=0 :  x = π/3
k=1 :  x = π
k=2 :  x = 5π/3