Cosine in terms of Sine

Theorem
Let $\theta$ be an angle.

Then:

where $\sin$ denotes the sine function and $\cos$ denotes the cosine function.

Proof
We also have that:
 * In quadrant I, and quadrant IV, $\cos \theta > 0$
 * In quadrant II, and quadrant III, $\cos \theta < 0$.

Also see

 * Trigonometric Functions in terms of each other