pi (π), the ratio of a circle's circumference to its diameter, is often written as 3.141592653. The decimal doesn't repeat, but you can estimate it using the Leibniz formula for pi:

Alternate adding and subtracting fractions with a numerator of 4 and a denominator of the subsequent odd number, starting with 1:

pi = (4/1) - (4/3) + (4/5) - (4/7) + (4/9) - (4/11) ...

The series is infinite - it goes on forever – and gets more precise with each additional term.