Find all to satisfy the equations in each of the following parts.

- .
- .
- .
- .

We use the result of the previous exercise (Section 2.8, Exercise 1), and the fundamental properties of the sine and cosine given in section 2.5-2.7 of Apostol in the following.

- , using fundamental property 2 (), and the -periodicity of sine.
- , using Theorem 2.3, part (d) (Apostol, Section 2.5).
- using part (a). So, .
- using fundamental property 2 () and the -periodicity of the cosine function.