Find the solutions of the equation

Question

Find the solutions of the equation that are in the interval [0, 2𝜋). (Enter your answers as a comma-separated list. If there is no solution, enter NO SOLUTION.)

cos u + cos 2u = 0

Answer 1

cosu + cos2u = 0; 2cos²u + cosu - 1 = 0; (2cosu - 1)(cosu + 1) = 0; cosu = 1/2 u = π/3; u = 5π/3

cosu = - 1; u = π

Answer: π/3, π, 5π/3