The formula can be derived by combining the equations [MATH]C = 2\pi r[/MATH] and [MATH]K = \pi r^2[/MATH], eliminating [MATH]r[/MATH] between them. Just solve the second equation for [MATH]r[/MATH], then put that into the first equation.
The last equation is poorly written; inline, it should be K = C2/(4 Pi), so that it means [MATH]K = \frac{C^2}{4\pi}[/MATH]. The final formula, [MATH]K = \frac{Cr}{2}[/MATH], is one of my favorites, showing that the area of a circle is the same as the area of a triangle with base [MATH]C[/MATH] and height [MATH]r[/MATH].