What is the cosine of an angle in the unit circle if the sine is negative?
In the unit circle, if the $\sin(\theta)$ is negative, it means that the angle $\theta$ is in the third or fourth quadrant.
In both of these quadrants, the sine value is negative.
Cosine values in these quadrants can be positive (fourth quadrant) or negative (third quadrant).
Therefore, the $\cos(\theta)$ can be expressed as:
$$\cos(\theta) = \pm\sqrt{1 – \sin^2(\theta)}$$