Assume and are two plane figures that are similar. Assume that is a segment or arc of that corresponds to a segment or arc of , and that is a segment or arc of that corresponds to a segment or arc of . Then

In particular, the ratio of the circumference *C* to the diameter *d*
is exactly the same for every circle. We can then *define*
to equal this ratio. That is to say, we *define* .
As a consequence, .

Wed Apr 21 08:17:28 EDT 1999