Equation of Circle/Parametric

Theorem
The equation of a circle with radius $R$ and center $\left({a, b}\right)$ expressed as a parametric equation
 * $x = a + R \cos t, y = b + R \sin t$

Proof
Let the point $\left({x, y}\right)$ satisfy the equations:
 * $x = a + R \cos t$
 * $y = b + R \sin t$

The distance between this point and the center of the circle is:
 * $\sqrt{\left({\left({a + R \cos t}\right) - a}\right)^2 + \left({\left({b + R \sin t}\right) - b}\right)^2}$

by the Distance Formula.

This simplifies to:
 * $\sqrt{R^2 \cos^2 t + R^2 \sin^2 t} = R \sqrt{\cos^2 t + \sin^2 t}$

Then by Sum of Squares of Sine and Cosine, this distance equals $R$.

Therefore the distance between points satisfying the equation and the center is constant and equal to the radius.