# What Is Pi?

## What Is Pi?

Pi is mathematical constant, denoted by the symbol π.

Pi is approximately equal to: ### Dictionary Definition

The Oxford English Dictionary defines pi as "a transcendental number which is equal to the ratio of the circumference of a circle to the diameter, having a decimal form beginning 3.14159265…, and commonly approximated to 227. Usually written π."

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## Pi Is the Ratio of Circumference to Diameter

Pi is the ratio of a circle's circumference to its diameter. In this formula, C is the circumference and d is the diameter.

This ratio is constant for all circles.

## Why Is Pi Useful?

### Geometry

Pi is useful in the geometry of circular shapes.

• The circumference of a circle with radius r is 2πr.

• The area of a circle with radius r is πr2.

• The surface area of a circle with radius r is 4πr2.

• The volume of a sphere with radius r is 4/3πr3.

Cones and cylinders also contain circles. Pi is useful in finding their volumes.

### Trigonometry

Pi is found in trigonometry because the sine, cosine and tangent functions can be defined from a unit circle.

Sine and cosine functions can be used to describe signals, like alternating currents and radio waves. This means π is often used when analysing signals, such as finding the power coming from a mains supply.