Equation of Circle/Polar/Corollary
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Corollary to Equation of Circle
Let $C$ be a circle whose radius is $R$.
Let $C$ be aligned in a polar coordinate frame such that its center is at the origin.
Then the equation of a $C$ is given by:
- $r = R$
Proof
From Equation of Circle: Polar Form, we have a circle whose center is at $\polar {r_0, \varphi}$ whose radius is $R$ is:
- $r^2 - 2 r r_0 \, \map \cos {\theta - \varphi} + \paren {r_0}^2 = R^2$
So, when $\polar {r_0, \varphi} = \polar {0, 0}$:
\(\ds \sqrt {r^2 + 0^2 - 2 r \cdot 0 \cdot \map \cos {\theta - 0} }\) | \(=\) | \(\ds R\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds \sqrt{r^2}\) | \(=\) | \(\ds R\) | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds r\) | \(=\) | \(\ds R\) |
$\blacksquare$