Daniel B. answered 12/08/22
A retired computer professional to teach math, physics
Case 1: cos(2x) = 0
Any value of x satisfying cos(2x)=0 also satisfies the given equation.
cos(2x) = 0
2x = π/2 + kπ for any integer k
x = π/4 + kπ/2 for any integer k
Case 2: cos(2x) ≠ 0
Then we can divide the given equation by cos(2x).
tan(3x) = 1
3x = π/4 + kπ for any integer k
x = π/12 + kπ/3 for any integer k
So there are the above two sets of solutions.