1. The problem is to find the formula for the circumference of a circle. 2. The circumference is the distance around the circle. 3. The formula for circumference $C$ is given by: $$C = 2\pi r$$ where $r$ is the radius of the circle and $\pi$ (pi) is approximately 3.14159. 4. Alternatively, if you know the diameter $d$ (which is twice the radius), the formula is: $$C = \pi d$$ 5. This formula helps you calculate the perimeter of any circle if you know its radius or diameter.