Unit circle
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The unit circle is a concept of mathematics (used in several contexts, especially in trigonometry). In essence, this is a circle constituted by all points that have Euclidean distance 1 from the origin (0,0) in a two-dimensional coordinate system. It is denoted by S¹.

The equation defining the points (x, y) of the unit circle is

One may also use other notions of "distance" to define other "unit circles"; see the article on normed vector space for examples.

Trigonometric functions in the unit circle

In a unit circle, several interesting things relating to trigonometric functions may be defined, with the given notation:

A point on the unit circle, pointed to by a certain vector from the origin with the angle from the -axis has the coordinates:

The equation of the circle above also immediately gives us the well-known "trigonometric 1":

The unit circle also gives an intuitive way of realizing that sine and cosine are periodic functions, with the identity

and for any integer k.

This identity comes from the fact that (x,y) coordinates remain the same after the angle t is increased or decreased by one revolution in the circle (2π). The notion of sine, cosine, and other trigonometric functions only makes sense with angles more than zero or less than π/2 when working with right triangles, but in the unit circle, angles outside this range have sensible, intuitive meanings.

See also

• Trigonometric function
• Angle measure

The original article can be found at: http://en.wikipedia.org/wiki/Unit_circle.
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