Sine
In mathematics, sine and cosine are trigonometric functions of an angle. The sine and cosine of an acute angle are defined in the context of a right triangle: the sine of an acute angle of a right triangle is the ratio of the length of the side opposite that angle to the length of the longest side of the triangle, the hypotenuse; the cosine is the ratio of the length of the adjacent side to that of the hypotenuse. For an angle
θ
{\displaystyle \theta }
, the sine and cosine functions are denoted as
sin
(
θ
)
{\displaystyle \sin(\theta )}
and
cos
(
θ
)
{\displaystyle \cos(\theta )}
.
The definitions of sine and cosine have been extended to any real value in terms of the lengths of certain line segments in a unit circle. More modern definitions express the sine and cosine as infinite series, or as the solutions of certain differential equations, allowing their extension to arbitrary positive and negative values and even to complex numbers.
The sine and cosine functions are commonly used to model periodic phenomena such as sound and light waves, the position and velocity of harmonic oscillators, sunlight intensity and day length, and average temperature variations throughout the year. They can be traced to the jyā and koṭi-jyā functions used in Indian astronomy during the Gupta period.
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