# How do you write -1 in trigonometric form?

(r,theta)=color(red)(""(1,pi))
$- 1$ in the Complex plane is a point on the negative X-axis (with no "imaginary" component).
The negative X-axis is at an angle of $\pi$ radians (or ${180}^{\circ}$) from the standard base, the positive X-axis.
$- 1$ is a distance of $1$ unit from the origin and this is what you use as the radius.