## Thinking Mathematically (6th Edition)

$\pi$ d
The circumference, C, of a circle with diameter d is given by the formula $\pi$d. The diameter, d, is a line segment through the center whose endpoints both lie on the circle, and the length of the diameter is twice the length of the radius. (d = 2r) As we know, C = 2$\pi$r Therefore, by using d =2r, we know that C = $\pi$d