# Difference between revisions of "Pi"

## Definition

Pi is an irrational number. It is the ratio of a circle's circumference, or perimeter, to its diameter. It is roughly equal to 3.141592653. The number pi is usually only useful when dealing with circles, spheres, and discs. The fractional approximation for pi (not exact) can be $\frac{22}{7}$. The exact formula for pi is $4\left( \sum_{i = 0}^\infty (-1)^i \left(\frac{1}{2n+1}\right)\right)$