Question

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