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 ^{22}⁄_{7}. Usually written π."
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 πr^{2}.

The surface area of a circle with radius r is 4πr^{2}.

The volume of a sphere with radius r is ^{4}/_{3}πr^{3}.
Cones and cylinders also contain circles. Pi is useful in finding their volumes.

The volume of a cone with height h and base radius r is ^{1}/_{3}πr^{2}h.

The volume of a cylinder with height h and base radius r is πr^{2}h.
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.