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{{ printedBook.courseTrack.name }} {{ printedBook.name }} The number pi is a constant defined as the ratio between the circumference and the diameter of a circle. This ratio is the same in every circle. Pi is usually written with the Greek letter $\pi.$ $\begin{gathered} \pi=3.1415926\ldots \end{gathered}$ Note that since $\pi$ is an irrational number, its value is often rounded to $3.14$ to make calculations easier.