An equation involving trigonometrical function is called a trigonometrical equation. To solve these equations, you are required to find all replacements for the variable θ that make the equations true.
A solution of a trigonometrical equation is the value of the unknown angle that satisfies the equation. A trigonometrical equation may have infinite number of solutions. The solution in which the absolute value of the angle is the least is called principal solution.
The expression involving integer n which gives all solutions of a trigonometrical equation is called the general solution.