Respuesta :

ghanami
hello : 
by idetity : 1+cos(2a) = 2cos²(a)
let : a = θ/2
1+cos(2×θ/2)=2cos²(θ/2)
1+cos(θ) = 2cos²(θ/2)
solve :  2 cos^2 (θ/2) - 3 cos(θ) = 0   ......(1)
(1) : 1+cos(θ)-3 cos(θ) =0
2cos(θ)  = 1 

cos(θ) = 1/2
in : [0, 2π[ 
( θ = π/3) or (θ = 5π/3)