.
Actually, tan(x) has a basic period from 0 to .
The interval from 0 to is TWO basic periods of tan(x).
Therefore, to simplify the problem, you could find the solutions to the given equation in the interval [0,) first,
and then to extend (to translate) the roots from this interval [0,) to one interval forward.
In the basic interval [0,), the roots are 0 and .
After moving forward one period, two other roots are added to the set of solutions, and .
Solved.
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Generally speaking, in Math it is not a good practice / (is not a good style)
to ask about the solutions of the equation
f(x) = 0,
where f(x) is periodical function with the period T, in two-period interval [0,2T).
It is not a good style.
Such questions, as a rule, go about one single period interval [0,T).
Otherwise, questions about mathematical competence of the author may arise.