# Area between Radii and Curve in Polar Coordinates

## Theorem

Let $C$ be a curve expressed in polar coordinates $\left\langle{r, \theta}\right\rangle$ as:

- $r = g \left({\theta}\right)$

where $g$ is a real function.

Let $\theta = \theta_a$ and $\theta = \theta_b$ be the two rays from the pole at angles $\theta_a$ and $\theta_b$ to the polar axis respectively.

Then the area $\mathcal A$ between $\theta_a$, $\theta_b$ and $C$ is given by:

- $\displaystyle \mathcal A = \int \limits_{\theta \mathop = \theta_a}^{\theta \mathop = \theta_b} \frac {\left({g \left({\theta}\right)}\right)^2 \, \mathrm d \theta} 2$

as long as $\left({g \left({\theta}\right)}\right)^2$ is integrable.

## Proof