Question 600086
Since cos(2u) = 2*cos^2(u) - 1, we can write the equation as
cos(u) - (2*cos^2(u) - 1) = 0
2*cos^2(u) - cos(u) - 1 = 0
Factor:
(2*cos(u)+1)(cos(u)-1) = 0
This gives cos(u) = -1/2 and cos(u) = 1
On the interval 0 <= u < 2{{{pi}}}, the solutions are u = 0, {{{2*pi/3}}} and {{{4*pi/3}}}