Question 963379
.
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 2u − cos u = 0
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        The solution and the answer in the post by @lwsshak3 both are incorrect.

        I came to bring a correct solution.



cos(2u) −   cos u  = 0
cos^2(u) - sin^2(u) - cosu = 0
cos^2(u) - 1 + cos^2(u) - cosu = 0
2cos^2(u) -  cosu - 1 = 0
(2cosu+1)(cosu-1)=0
..
2cos(u)+1 = 0
cos(u) = -1/2
u = 2π/3, 4π/3
or
cos(u) - 1 = 0
cos(u) = 1
u = 0.


Solved correctly.