\begin{definition}[Definition:Cotangent/Definition from Circle/Third Quadrant]
Consider a unit circle $C$ whose center is at the origin of a cartesian plane.
:500px
Let $P$ be the point on $C$ in the third quadrant such that $\theta$ is the angle made by $OP$ with the $x$-axis.
Let a tangent line be drawn to touch $C$ at $A = \tuple {0, 1}$.
Let $OP$ be produced to meet this tangent line at $B$.
Then the '''cotangent''' of $\theta$ is defined as the length of $AB$.
Category:Definitions/Cotangent Function
\end{definition}