Unit circle
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 twodimensional
coordinate system. It is denoted by
S^{1}.
Illustration of a unit circle.
t is an
angle measure.
Image:UnitCircle.png
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 wellknown "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 n.
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.