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< | <!--Hey can we get a picture (including the tangent values) of the points of interest on the unit circle with the tangent values shown also?-->[[Image:Unit circle.svg|Unit circle|right|thumb|186px|Illustration of a unit circle. The variable ''t'' is an [[angle]] measure.]] | ||
In [[mathematics]], a '''unit circle''' is a [[circle]] with a [[radius]] of [[1 (number)|one]]. Frequently, especially in [[trigonometry]], the unit circle is the circle of radius one centered at the origin (0, 0) in the [[Cartesian coordinate system]] in the [[Euclidean plane]]. The unit circle is often denoted ''S''<sup>1</sup>; the generalization to higher dimensions is the [[unit sphere]]. | |||
If (''x'', ''y'') is a point on the unit circle, then |''x''| and |''y''| are the lengths of the legs of a [[right triangle]] whose hypotenuse has length 1. Thus, by the [[Pythagorean theorem]], ''x'' and ''y'' satisfy the equation | |||
:<math>x^2 + y^2 = 1.</math> | |||
Since ''x''² = (−''x'')² for all ''x'', and since the reflection of any point on the unit circle about the ''x''- or ''y''-axis is also on the unit circle, the above equation holds for all points (''x'', ''y'') on the unit circle, not only those in the first quadrant. | |||
One may also use other notions of "distance" to define other "unit circles", such as the [[Riemannian circle]]; see the article on [[norm (mathematics)|mathematical norms]] for additional examples. | |||
==In the complex plane== | |||
The unit circle can be considered as the set of [[complex numbers]] ''z'' of the form | |||
:<math> z = \,\mathrm{e}^{i t}\, = \cos(t) + i \sin(t) \,</math> | |||
for all ''t''. This relation is [[Euler's formula]]. | |||
==Trigonometric functions on the unit circle== | |||
[[Image:Circle-trig6.svg|right|thumb|300px|''All'' of the trigonometric functions of the angle ''θ'' can be constructed geometrically in terms of a unit circle centered at ''O''.]] | |||
[[File:Periodic sine.PNG|thumb|Sine function on unit circle (top) and its graph (bottom)]] | |||
The [[trigonometric function]]s cosine and sine may be defined on the unit circle as follows. If (''x'', ''y'') is a point of the unit circle, and if the ray from the origin (0, 0) to (''x'', ''y'') makes an [[angle]] ''t'' from the positive ''x''-axis, (where counterclockwise turning is positive), then | |||
:<math>\cos(t) = x \,\!</math> | |||
:<math>\sin(t) = y. \,\!</math> | |||
The equation ''x''<sup>2</sup> + ''y''<sup>2</sup> = 1 gives the relation | |||
:<math> \cos^2(t) + \sin^2(t) = 1. \,\!</math> | |||
The unit circle also demonstrates that [[sine]] and [[cosine]] are [[periodic function]]s, with the identities | |||
:<math>\cos t = \cos(2\pi k+t) \,\!</math> | |||
:<math>\sin t = \sin(2\pi k+t) \,\!</math> | |||
for any [[integer]] ''k''. | |||
Triangles constructed on the unit circle can also be used to illustrate the periodicity of the trigonometric functions. First, construct a radius OA from the origin to a point P(''x''<sub>1</sub>,''y''<sub>1</sub>) on the unit circle such that an angle ''t'' with 0 < ''t'' < π/2 is formed with the positive arm of the ''x''-axis. Now consider a point Q(''x''<sub>1</sub>,0) and line segments PQ <math>\perp</math> OQ. The result is a right triangle ΔOPQ with ∠QOP = ''t''. Because PQ has length ''y''<sub>1</sub>, OQ length ''x''<sub>1</sub>, and OA length 1, sin(''t'') = ''y''<sub>1</sub> and cos(''t'') = ''x''<sub>1</sub>. Having established these equivalences, take another radius OR from the origin to a point R(−''x''<sub>1</sub>,''y''<sub>1</sub>) on the circle such that the same angle ''t'' is formed with the negative arm of the ''x''-axis. Now consider a point S(''−x<sub>1</sub>'',0) and line segments RS <math>\perp</math> OS. The result is a right triangle ΔORS with ∠SOR = ''t''. It can hence be seen that, because ∠ROQ = π−''t'', R is at (cos(π−''t''),sin(π−''t'')) in the same way that P is at (cos(''t''),sin(''t'')). The conclusion is that, since (−''x''<sub>1</sub>,''y''<sub>1</sub>) is the same as (cos(π−''t''),sin(π−''t'')) and (''x''<sub>1</sub>,''y''<sub>1</sub>) is the same as (cos(''t''),sin(''t'')), it is true that sin(''t'') = sin(π−''t'') and −cos(''t'') = cos(π−''t''). It may be inferred in a similar manner that tan(π−''t'') = −tan(''t''), since tan(''t'') = ''y''<sub>1</sub>/''x''<sub>1</sub> and tan(π−''t'') = ''y''<sub>1</sub>/(−''x''<sub>1</sub>). A simple demonstration of the above can be seen in the equality sin(π/4) = sin(3π/4) = 1/sqrt(2). | |||
When working with right triangles, sine, cosine, and other trigonometric functions only make sense for angle measures more than zero and less than π/2. However, when defined with the unit circle, these functions produce meaningful values for any [[real number|real]]-valued angle measure – even those greater than 2π. In fact, all six standard trigonometric functions – sine, cosine, tangent, cotangent, secant, and cosecant, as well as archaic functions like [[versine]] and [[exsecant]] – can be defined geometrically in terms of a unit circle, as shown at right. | |||
Using the unit circle, the values of any trigonometric function for many angles other than those labeled can be calculated without the use of a calculator by using the [[Trigonometric identity#Angle_sum_and_difference_identities|Sum and Difference Formulas]]. | |||
[[Image:Unit circle angles color.svg|thumb|300px|left|The unit circle, showing [[Exact trigonometric constants|coordinates of certain points]]]] <!--Get a picture with the tangent values show--> | |||
==Circle group== | |||
[[Complex number]]s can be identified with points in the [[Euclidean plane]], namely the number ''a'' + ''bi'' is identified with the point (''a'', ''b''). Under this identification, the unit circle is a [[group (mathematics)|group]] under multiplication, called the [[circle group]]. On the plane multiplication by <math>\cos \theta + i \sin \theta</math> gives a counterclockwise rotation by θ. This group has important applications in mathematics and science.{{Such as?}} | |||
==Complex dynamics== | |||
{{Main|Complex dynamics}} | |||
[[Image:Erays.png|right|thumb|Unit circle in complex dynamics]] | |||
[[Julia set]] of [[Dynamical system (definition)|discrete nonlinear dynamical system]] with [[evolution function]]: | |||
:<math>f_0(x) = x^2 \,</math> | |||
is a unit circle. It is a simplest case so it is widely used in study of dynamical systems. | |||
==See also== | |||
*[[Angle|Angle measure]] | |||
*[[Circle group]] | |||
*[[Pythagorean trigonometric identity]] | |||
*[[Riemannian circle]] | |||
*[[Unit disc]] | |||
*[[Unit hyperbola]] | |||
*[[Unit square]] | |||
*[[Z-transform]] | |||
==External links== | |||
{{Wikibooks|Trigonometry/The unit circle}} | |||
{{Wiktionary|unit circle}} | |||
*{{mathworld | urlname = UnitCircle | title = Unit circle}} | |||
*[http://www.dudefree.com/unitcircle/ Flash animation for learning the unit circle] | |||
*[http://glab.trixon.se/ GonioLab]: Visualization of the unit circle, trigonometric and hyperbolic functions | |||
[[Category:Circles]] | |||
[[Category:One]] | |||
[[Category:Trigonometry]] | |||
[[Category:Fourier analysis]] | |||
[[Category:Analytic geometry]] | |||
Revision as of 05:15, 28 January 2014
In mathematics, a unit circle is a circle with a radius of one. Frequently, especially in trigonometry, the unit circle is the circle of radius one centered at the origin (0, 0) in the Cartesian coordinate system in the Euclidean plane. The unit circle is often denoted S1; the generalization to higher dimensions is the unit sphere.
If (x, y) is a point on the unit circle, then |x| and |y| are the lengths of the legs of a right triangle whose hypotenuse has length 1. Thus, by the Pythagorean theorem, x and y satisfy the equation
Since x² = (−x)² for all x, and since the reflection of any point on the unit circle about the x- or y-axis is also on the unit circle, the above equation holds for all points (x, y) on the unit circle, not only those in the first quadrant.
One may also use other notions of "distance" to define other "unit circles", such as the Riemannian circle; see the article on mathematical norms for additional examples.
In the complex plane
The unit circle can be considered as the set of complex numbers z of the form
for all t. This relation is Euler's formula.
Trigonometric functions on the unit circle
The trigonometric functions cosine and sine may be defined on the unit circle as follows. If (x, y) is a point of the unit circle, and if the ray from the origin (0, 0) to (x, y) makes an angle t from the positive x-axis, (where counterclockwise turning is positive), then
The equation x2 + y2 = 1 gives the relation
The unit circle also demonstrates that sine and cosine are periodic functions, with the identities
for any integer k.
Triangles constructed on the unit circle can also be used to illustrate the periodicity of the trigonometric functions. First, construct a radius OA from the origin to a point P(x1,y1) on the unit circle such that an angle t with 0 < t < π/2 is formed with the positive arm of the x-axis. Now consider a point Q(x1,0) and line segments PQ OQ. The result is a right triangle ΔOPQ with ∠QOP = t. Because PQ has length y1, OQ length x1, and OA length 1, sin(t) = y1 and cos(t) = x1. Having established these equivalences, take another radius OR from the origin to a point R(−x1,y1) on the circle such that the same angle t is formed with the negative arm of the x-axis. Now consider a point S(−x1,0) and line segments RS OS. The result is a right triangle ΔORS with ∠SOR = t. It can hence be seen that, because ∠ROQ = π−t, R is at (cos(π−t),sin(π−t)) in the same way that P is at (cos(t),sin(t)). The conclusion is that, since (−x1,y1) is the same as (cos(π−t),sin(π−t)) and (x1,y1) is the same as (cos(t),sin(t)), it is true that sin(t) = sin(π−t) and −cos(t) = cos(π−t). It may be inferred in a similar manner that tan(π−t) = −tan(t), since tan(t) = y1/x1 and tan(π−t) = y1/(−x1). A simple demonstration of the above can be seen in the equality sin(π/4) = sin(3π/4) = 1/sqrt(2).
When working with right triangles, sine, cosine, and other trigonometric functions only make sense for angle measures more than zero and less than π/2. However, when defined with the unit circle, these functions produce meaningful values for any real-valued angle measure – even those greater than 2π. In fact, all six standard trigonometric functions – sine, cosine, tangent, cotangent, secant, and cosecant, as well as archaic functions like versine and exsecant – can be defined geometrically in terms of a unit circle, as shown at right.
Using the unit circle, the values of any trigonometric function for many angles other than those labeled can be calculated without the use of a calculator by using the Sum and Difference Formulas.

Circle group
Complex numbers can be identified with points in the Euclidean plane, namely the number a + bi is identified with the point (a, b). Under this identification, the unit circle is a group under multiplication, called the circle group. On the plane multiplication by gives a counterclockwise rotation by θ. This group has important applications in mathematics and science.Template:Such as?
Complex dynamics
Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church.

Julia set of discrete nonlinear dynamical system with evolution function:
is a unit circle. It is a simplest case so it is widely used in study of dynamical systems.
See also
- Angle measure
- Circle group
- Pythagorean trigonometric identity
- Riemannian circle
- Unit disc
- Unit hyperbola
- Unit square
- Z-transform
External links
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- GonioLab: Visualization of the unit circle, trigonometric and hyperbolic functions