473. Suppose it has already been preliminarily proven that the length of a circle is proportional to its radius. What else needs to be done to derive the formula for the circumference of a circle?
473. When it has been proven that the length of the circumference $C$ is proportional to the radius $r$, this establishes the formula $C=2 \pi r$, since this formula merely expresses the stated proportionality, with the coefficient of proportionality denoted by $2 \pi$. The numerical value of the symbol $\pi$ is determined by later theorems.