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trigonometric functions 三角函数


  In mathematics , the trigonometric functions (also called the circular functions) are functions  of an angle . They relate the angles of a triangle  to the lengths of its sides. Trigonometric functions are important in the study of triangles and modeling periodic phenomena, among many other applications.
  The most familiar trigonometric functions are the sine, cosine , and tangent . In the context of the standard unit circle (a circle with radius  1 unit), where a triangle is formed by a ray originating at the origin and making some angle with the x-axis, the sine of the angle gives the length of the y-component (the opposite to the angle or the rise) of the triangle, the cosine gives the length of the x-component (the adjacent of the angle or the run), and the tangent function gives the slope (y-component divided by the x-component). More precise definitions are detailed below. Trigonometric functions are commonly defined as ratios  of two sides of a right triangle containing the angle, and can equivalently be defined as the lengths of various line segments from a unit circle . More modern definitions express them as infinite series  or as solutions of certain differential equations , allowing their extension to arbitrary positive and negative values and even to complex numbers.
  Trigonometric functions have a wide range of uses including computing unknown lengths and angles in triangles  (oftenright triangles ). In this use, trigonometric functions are used, for instance, in navigation, engineering, and physics. A common use in elementary physics is resolving a vector into Cartesian  coordinates. The sine and cosine functions are also commonly used to model periodic function phenomena such as sound and light waves, the position and velocity of harmonic oscillators, sunlight intensity and day length, and average temperature variations through the year.
  In modern usage, there are six basic trigonometric functions, tabulated here with equations that relate them to one another. Especially with the last four, these relations are often taken as the definitions of those functions, but one can define them equally well geometrically, or by other means, and then derive these relations.