<Dd> C = π ⋅ d = 2 π ⋅ r . (\ displaystyle (C) = \ pi \ cdot (d) = 2 \ pi \ cdot (r). \!) </Dd> <P> The use of the mathematical constant π is ubiquitous in mathematics, engineering, and science . </P> <P> In Measurement of a Circle written circa 250 BCE, Archimedes showed that this ratio (C / d, since he did not use the name π) was greater than 310 / 71 but less than 31 / 7 by calculating the perimeters of an inscribed and a circumscribed regular polygon of 96 sides . This method for approximating π was used for centuries, obtaining more accuracy by using polygons of larger and larger number of sides . The last such calculation was performed in 1630 by Christoph Grienberger who used polygons with 10 sides . </P> <P> Circumference is used by some authors to denote the perimeter of an ellipse . There is no general formula for the circumference of an ellipse in terms of the semi-major and semi-minor axes of the ellipse that uses only elementary functions . However, there are approximate formulas in terms of these parameters . One such approximation, due to Euler (1773), for the canonical ellipse, </P>

Who discovered the formula for the circumference of a circle