Trigonometric Function

In Mathematics , the trigonometric functions (also called circular functions) are Function s of an Angle . They are important in the study of Triangles and modeling Periodic Phenomena , among many other applications. Trigonometric functions are commonly defined as Ratio s 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 Equation s, allowing their extension to arbitrary positive and negative values and even to Complex Number s.

In modern usage, there are six basic trigonometric functions, which are tabulated here along with equations relating them to one another. Especially in the case of 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.

HISTORY

See Also: History of trigonometric functions

The notion that there should be some standard correspondence between the length of the sides of a triangle and the angles of the triangle comes as soon as one recognizes that similar triangles maintain the same ratios between their sides. That is, for any similar triangle the ratio of the hypotenuse (for example) and another of the sides remains the same. If the hypotenuse is twice as long, so are the sides. It is just these ratios that the trigonometric functions express.

Trigonometric functions were studied by Hipparchus of Nicaea (180-125 BC), Ptolemy of Egypt (90–180 AD), Aryabhata (476–550), Varahamihira , Brahmagupta , , Abū Al-Wafā' Al-Būzjānī , Omar Khayyam , Bhāskara II , Nasir Al-Din Al-Tusi , Ghiyath Al-Kashi (14th century), Ulugh Beg (14th century), Regiomontanus (1464), Rheticus , and Rheticus' student Valentin Otho.

Madhava Of Sangamagramma (c. 1400) made early strides in the Analysis of trigonometric functions in terms of Infinite Series . Leonhard Euler 's ''Introductio in analysin infinitorum'' (1748) was mostly responsible for establishing the analytic treatment of trigonometric functions in Europe, also defining them as infinite series and presenting " Euler's Formula ", as well as the near-modern abbreviations ''sin., cos., tang., cot., sec.,'' and ''cosec.''

A few functions were common historically (and appeared in the earliest tables), but are now seldom used, such as the Chord (crd(θ) = 2 sin(θ/2)), the Versine (versin(θ) = 1 − cos(θ) = 2 sin&²(θ/2)), the Haversine (haversin(θ) = versin(θ) / 2 = sin&²(θ/2)), the Exsecant (exsec(θ) = sec(θ) − 1) and the Excosecant (excsc(θ) = exsec(π/2 − θ) = csc(θ) − 1). Many more relations between these functions are listed in the article about Trigonometric Identities .

RIGHT TRIANGLE DEFINITIONS

always includes a 90° (π/2 radians) angle, here labeled C. Angles A and B may vary. Trigonometric functions specify the relationships among side lengths and interior angles of a right triangle.]]

In order to define the trigonometric functions for the angle ''A'', start with an arbitrary Right Triangle that contains the angle ''A'':

We use the following names for the sides of the triangle:

The '' Hypotenuse '' is the side opposite the right angle, or defined as the longest side of a right-angled triangle, in this case h.

The ''opposite side'' is the side opposite to the angle we are interested in, in this case a.

The ''adjacent side'' is the side that is in contact with the angle we are interested in and the right angle, hence its name. In this case the adjacent side is b.

All triangles are taken to exist in the Euclidean Plane so that the inside angles of each triangle sum to π Radian s (or 180 ° ); therefore, for a right triangle the two non-right angles are between zero and π/2 radians (or 90 ° ). The reader should note that the following definitions, strictly speaking, only define the trigonometric functions for angles in this range. We extend them to the full set of real arguments by using the Unit Circle , or by requiring certain symmetries and that they be Periodic Function s.

1) The sine of an angle is the ratio of the length of the opposite side to the length of the hypotenuse. In our case
:

\sin A = rac {	extrm{opposite}} {	extrm{hypotenuse}} = rac {a} {h}

.
Note that this ratio does not depend on the particular right triangle chosen, as long as it contains the angle ''A'', since all those triangles are Similar .

The set of zeroes of sine (i.e., the values of

x

for which

\sin x =0

) is

The set of zeros of cosine is

The set of zeroes of tangent is

5) The secant sec(''A'') is the Multiplicative Inverse of cos(''A''), i.e. the ratio of the length of the hypotenuse to the length of the adjacent side:
:

\sec A = rac {	extrm{hypotenuse}} {	extrm{adjacent}} = rac {h} {b}

.

6) The cotangent cot(''A'') is the Multiplicative Inverse of tan(''A''), i.e. the ratio of the length of the adjacent side to the length of the opposite side:
:

\cot A = rac {	extrm{adjacent}} {	extrm{opposite}} = rac {b} {a}

.

Mnemonics

There are a number of Mnemonic s for the above definitions; for example, ''SOHCAHTOA'' (sounds like "soak a toe-a" or "sock-a toe-a"). It means:

This can also be remembered as rhymes such as "Some '''O'''ld '''H'''ippie '''C'''aught '''A'''nother '''H'''ippie '''T'''rippin' '''O'''n '''A'''cid or; "Some '''O'''ld '''H'''ags '''C'''an't '''A'''lways '''H'''ide '''T'''heir '''O'''ld '''A'''ge" with the initials, in triplets, forming the ratios for sine, cosine, and tangent, respectively.

Slope definitions

Equivalent to the right-triangle definitions, the trigonometric functions can be defined in terms of the ''rise'', ''run'', and '' Slope '' of a line segment relative to some horizontal line. The slope is commonly taught as "rise over run" or rise/run. The three main trigonometric functions are commonly taught in the order sine, cosine, tangent. With a Unit Circle , the following correspondense of definitions exists:

# Sine is first, rise is first. Sine takes an angle and tells the rise.
# Cosine is second, run is second. Cosine takes an angle and tells the run.
# Tangent is the slope formula that combines the rise and run. Tangent takes an angle and tells the slope.

This shows the main use of tangent and arctangent: converting between the two ways of telling the slant of a line, ''i.e.,'' angles and slopes. (Note that the arctangent or "inverse tangent" is not to be confused with the ''cotangent,'' which is cos divided by sin.)

While the radius of the circle makes no difference for the slope (the slope doesn't depend on the length of the slanted line), it does affect rise and run. To adjust and find the actual rise and run, just multiply the sine and cosine by the radius. For instance, if the circle has radius 5, the run at an angle of 1° is 5 cos(1°)

UNIT-CIRCLE DEFINITIONS

]]

The six trigonometric functions can also be defined in terms of the Unit Circle , the Circle of radius one centered at the origin. The unit circle definition provides little in the way of practical calculation; indeed it relies on right triangles for most angles. The unit circle definition does, however, permit the definition of the trigonometric functions for all positive and negative arguments, not just for angles between 0 and π/2 radians. It also provides a single visual picture that encapsulates at once all the important triangles. From the Pythagorean Theorem the equation for the unit circle is:
:

x^2 + y^2 = 1 \,

In the picture, some common angles, measured in radians, are given. Measurements in the counter clockwise direction are positive angles and measurements in the clockwise direction are negative angles. Let a line through the origin, making an angle of θ with the positive half of the ''x''-axis intersect the unit circle. The ''x''- and ''y''-coordinates of this point of intersection are equal to cos θ and sin θ, respectively. The triangle in the graphic enforces the formula; the radius is equal to the hypotenuse and has length 1, so we have sin θ = ''y''/1 and cos θ = ''x''/1. The unit circle can be thought of as a way of looking at an infinite number of triangles by varying the lengths of their legs but keeping the lengths of their hypotenuses equal to 1.

:

\cos x = 1 - rac{x^2}{2!} + rac{x^4}{4!} - rac{x^6}{6!} + \cdots = \sum_{n=0}^\infty rac{(-1)^nx^{2n}}{(2n)!}

These identities are often taken as the ''definitions'' of the sine and cosine function. They are often used as the starting point in a rigorous treatment of trigonometric functions and their applications (''e.g.,'' in Fourier Series ), since the theory of Infinite Series can be developed from the foundations of the Real Number System , independent of any geometric considerations. The Differentiability and Continuity of these functions are then established from the series definitions alone.

Other series can be found:Abramowitz; Weisstein.

<math> {}

x + rac{x^3}{3} + rac{2 x^5}{15} + rac{17 x^7}{315} + \cdots,

<math> {}

rac {1} {x} + rac {x} {6} + rac {7 x^3} {360} + rac {31 x^5} {15120} + \cdots,

<math> {}

1 + rac {x^2} {2} + rac {5 x^4} {24} + rac {61 x^6} {720} + \cdots,

<math> {}

rac {1} {x} - rac {x}{3} - rac {x^3} {45} - rac {2 x^5} {945} - \cdots,

\cos z \, = \, \sum_{n=0}^{\infty}rac{(-1)^{n}}{(2n)!}z^{2n} \, = \, {e^{i z} + e^{-i z} \over 2} = \cosh \left(i z
ight)

where ''i''^{2 = −1. Also, for purely real ''x'',

: $\cos x \, = \, \mbox{Re } (e^{i x})$

: $\sin x \, = \, \mbox{Im } (e^{i x})$

It is also known that exponential processes are intimately linked to periodic behavior.

DEFINITIONS VIA DIFFERENTIAL EQUATIONS

Both the sine and cosine functions satisfy the Differential Equation

: $y''=-y$ .

That is to say, each is the additive inverse of its own second derivative. Within the 2-dimensional Function Space ''V'' consisting of all solutions of this equation, the sine function is the unique solution satisfying the initial conditions ''y''(0) = 0 and ''y''′(0) = 1, and the cosine function is the unique solution satisfying the initial conditions ''y''(0) = 1 and ''y''′(0) = 0. Since the sine and cosine functions are linearly independent, together they form a Basis of ''V''. This method of defining the sine and cosine functions is essentially equivalent to using Euler's formula. (See Linear Differential Equation .) It turns out that this differential equation can be used not only to define the sine and cosine functions but also to prove the Trigonometric Identities for the sine and cosine functions. Further, the observation that sine and cosine satisfies $y''=-y$ means that they are Eigenfunction s of the second-derivative operator.

The tangent function is the unique solution of the nonlinear differential equation

: $y'=1+y^2$

satisfying the initial condition ''y''(0) = 0. There is a very interesting visual proof that the tangent function satisfies this differential equation; see Needham's ''Visual Complex Analysis.''Needham, p. ix.

The significance of radians

Radians specify an angle by measuring the length around the path of the unit circle and constitute a special argument to the sine and cosine functions. In particular, only those sines and cosines which map radians to ratios satisfy the differential equations which classically describe them. If an argument to sine or cosine in radians is scaled by frequency,

: $f(x) = \sin(kx); k
e 0, k
e 1 \,$

then the derivatives will scale by ''amplitude''.

: $f'(x) = k\cos(kx) \,$ .

Here, ''k'' is a constant that represents a mapping between units. If ''x'' is in degrees, then

: $k = rac{\pi}{180^\circ}$ .

This means that the second derivative of a sine in degrees satisfies not the differential equation

: $y'' = -y \,$ ,

but

: $y'' = -k^2y \,$ ;

similarly for cosine.

This means that these sines and cosines are different functions, and that the fourth derivative of sine will be sine again only if the argument is in radians.

IDENTITIES

Main article: List Of Trigonometric Identities .

Many identities exist which interrelate the trigonometric functions. Among the most frequently used is the Pythagorean identity, which states that for any angle, the square of the sine plus the square of the cosine is always 1. This is easy to see by studying a right triangle of hypotenuse 1 and applying the Pythagorean Theorem . In symbolic form, the Pythagorean identity reads,

: $\left(\sin x
ight)^2 + \left(\cos x
ight)^2 = 1$ ,

which is more commonly written with the exponent "two" next to the sine and cosine symbol:

: $\sin^2\left( x
ight) + \cos^2\left(x
ight) = 1$ .

In some cases the inner parentheses may be omitted.

Other key relationships are the sum and difference formulas, which give the sine and cosine of the sum and difference of two angles in terms of sines and cosines of the angles themselves. These can be derived geometrically, using arguments which go back to Ptolemy ; one can also produce them algebraically using Euler's formula.

: $\sin \left(x+y
ight)=\sin x \cos y + \cos x \sin y$

: $\cos \left(x+y
ight)=\cos x \cos y - \sin x \sin y$

: $\sin \left(x-y
ight)=\sin x \cos y - \cos x \sin y$

: $\cos \left(x-y
ight)=\cos x \cos y + \sin x \sin y$

When the two angles are equal, the sum formulas reduce to simpler equations known as the double-angle formulas.

These identities can also be used to derive the Product-to-sum Identities that were used in antiquity to transform the product of two numbers in a sum of numbers and greatly speed operations, much like the Logarithm Function .

For Integral s and Derivative s of trigonometric functions, see the relevant sections of Table Of Derivatives , Table Of Integrals , and List Of Integrals Of Trigonometric Functions .

Definitions using functional equations

In Mathematical Analysis , one can define the trigonometric functions using Functional Equation s based on properties like the sum and difference formulas. Taking as given these formulas and the Pythagorean identity, for example, one can prove that only two Real Function s satisfy those conditions. Symbolically, we say that there exists exactly one pair of real functions ''sin'' and ''cos'' such that for all real numbers ''x'' and ''y,'' the following equations hold:

: $\sin^2(x) + \cos^2(x) = 1,\,$

: $\sin(x+y) = \sin(x)\cos(y) + \cos(x)\sin(y),\,$

: $\cos(x+y) = \cos(x)\cos(y) - \sin(x)\sin(y),\,$

with the added condition that

: $0 < x\cos(x) < \sin(x) < x\ \mathrm{for}\ 0 < x < 1$ .

Other derivations, starting from other functional equations, are also possible, and such derivations can be extended to the complex numbers.

As an example, this derivation can be used to define Trigonometry In Galois Fields .

COMPUTATION

The computation of trigonometric functions is a complicated subject, which can today be avoided by most people because of the widespread availability of Computer s and Scientific Calculator s that provide built-in trigonometric functions for any angle. In this section, however, we describe more details of their computation in three important contexts: the historical use of trigonometric tables, the modern techniques used by computers, and a few "important" angles where simple exact values are easily found. (Below, it suffices to consider a small range of angles, say 0 to π/2, since all other angles can be reduced to this range by the periodicity and symmetries of the trigonometric functions.)

See Also: Generating trigonometric tables

Prior to computers, people typically evaluated trigonometric functions by Interpolating from a detailed table of their values, calculated to many Significant Figures . Such tables have been available for as long as trigonometric functions have been described (see History above), and were typically generated by repeated application of the half-angle and angle-addition Identities starting from a known value (such as sin(π/2)=1).

Modern computers use a variety of techniques.Kantabutra. One common method, especially on higher-end processors with Floating Point units, is to combine a Polynomial or Rational Approximation (such as Chebyshev Approximation , best uniform approximation, and Padé Approximation , and typically for higher or variable precisions, Taylor and Laurent Series ) with range reduction and a table lookup — they first look up the closest angle in a small table, and then use the polynomial to compute the correction.However, doing that while maintaining precision is nontrivial, and methods like Gal's accurate tables, Cody and Waite reduction, and Payne and Hanek reduction algorithms can be used. On simpler devices that lack Hardware Multiplier s, there is an algorithm called CORDIC (as well as related techniques) that is more efficient, since it uses only Shift s and additions. All of these methods are commonly implemented in Hardware for performance reasons.

For very high precision calculations, when series expansion convergence becomes too slow, trigonometric functions can be approximated by the Arithmetic-geometric Mean , which itself approximates the trigonometric function by the ( Complex ) Elliptic Integral . R. P. Brent, "Fast Multiple-Precision Evaluation of Elementary Functions", J. ACM 23, 242 (1976).

See Also: Exact trigonometric constants

Finally, for some simple angles, the values can be easily computed by hand using the Pythagorean Theorem , as in the following examples. In fact, the sine, cosine and tangent of any integer multiple of $\pi / 60$ Radians (3°) can be found Exactly By Hand .

Consider a right triangle where the two other angles are equal, and therefore are both $\pi / 4$ radians (45°). Then the length of side ''b'' and the length of side ''a'' are equal; we can choose $a = b = 1$ . The values of sine, cosine and tangent of an angle of $\pi / 4$ radians (45°) can then be found using the Pythagorean theorem:

: $c = \sqrt { a^2+b^2 } = \sqrt2$ .

Therefore:

: $\sin \left(\pi / 4
ight) = \sin \left(45^\circ
ight) = \cos \left(\pi / 4
ight) = \cos \left(45^\circ
ight) = {1 \over \sqrt2}$ ,

: $an \left(\pi / 4
ight) = an \left(45^\circ
ight) = = {1 \over \sqrt2} \cdot {\sqrt2 \over 1} = {\sqrt2 \over \sqrt2} = 1$ .

To determine the trigonometric functions for angles of π/3 radians (60 degrees) and π/6 radians (30 degrees), we start with an equilateral triangle of side length 1. All its angles are π/3 radians (60 degrees). By dividing it into two, we obtain a right triangle with π/6 radians (30 degrees) and π/3 radians (60 degrees) angles. For this triangle, the shortest side = 1/2, the next largest side =(√3)/2 and the hypotenuse = 1. This yields:

: $\sin \left(\pi / 6
ight) = \sin \left(30^\circ
ight) = \cos \left(\pi / 3
ight) = \cos \left(60^\circ
ight) = {1 \over 2}$ ,

: $\cos \left(\pi / 6
ight) = \cos \left(30^\circ
ight) = \sin \left(\pi / 3
ight) = \sin \left(60^\circ
ight) = {\sqrt3 \over 2}$ ,

: $an \left(\pi / 6
ight) = an \left(30^\circ
ight) = \cot \left(\pi / 3
ight) = \cot \left(60^\circ
ight) = {1 \over \sqrt3}$ .

INVERSE FUNCTIONS

See Also: inverse trigonometric function

The trigonometric functions are periodic, and hence not Injective , so strictly they don't have an Inverse Function . Therefore to define an inverse function we must restrict their domains so that the trigonometric function is Bijective . In the following, the functions on the left are ''defined'' by the equation on the right; these are not proved identities. The principal inverses are usually defined as:

: $\begin{matrix}

\mbox{for} & -rac{\pi}{2} \le y \le rac{\pi}{2},
& y = \arcsin(x) & \mbox{if} & x = \sin(y) \ \
\mbox{for} & 0 \le y \le \pi,
& y = \arccos(x) & \mbox{if} & x = \cos(y) \ \
\mbox{for} & -rac{\pi}{2} < y < rac{\pi}{2},
& y = \arctan(x) & \mbox{if} & x = an(y) \ \
\mbox{for} & -rac{\pi}{2} \le y \le rac{\pi}{2}, y
e 0,
& y = \arccsc(x) & \mbox{if} & x = \csc(y) \ \
\mbox{for} & 0 \le y \le \pi, y
e rac{\pi}{2},
& y = \arcsec(x) & \mbox{if} & x = \sec(y) \ \
\mbox{for} & 0 < y < \pi,
& y = \arccot(x) & \mbox{if} & x = \cot(y)

\end{matrix}$

For inverse trigonometric functions, the notations sin⁻¹ and cos⁻¹ are often used for arcsin and arccos, etc. When this notation is used, the inverse functions could be confused with the multiplicative inverses of the functions. The notation using the "arc-" prefix avoids such confusion, though "arcsec" can be confused with " Arcsecond ".

Just like the sine and cosine, the inverse trigonometric functions can also be defined in terms of infinite series. For example,

: $\arcsin z = z + \left( rac {1} {2}
ight) rac {z^3} {3} + \left( rac {1 \cdot 3} {2 \cdot 4}
ight) rac {z^5} {5} + \left( rac{1 \cdot 3 \cdot 5} {2 \cdot 4 \cdot 6 }
ight) rac{z^7} {7} + \cdots$

These functions may also be defined by proving that they are antiderivatives of other functions. The arcsine, for example, can be written as the following integral:

:

\arcsin\left(x
ight) =

In the animation on the right it can be seen that just a few terms already produce a fairly good approximation.

NOTES

REFERENCES

Abramowitz, Milton and Irene A. Stegun, '' Handbook Of Mathematical Functions With Formulas, Graphs, And Mathematical Tables '', Dover, New York. (1964). ISBN 0-486-61272-4.

Boyer, Carl B. , ''A History of Mathematics'', John Wiley & Sons, Inc., 2nd edition. (1991). ISBN 0-471-54397-7.

Joseph, George G., ''The Crest of the Peacock: Non-European Roots of Mathematics'', 2nd ed. Penguin Books , London. (2000). ISBN 0-691-00659-8.

Kantabutra, Vitit, "On hardware for computing exponential and trigonometric functions," ''IEEE Trans. Computers'' 45 (3), 328–339 (1996).

Maor, Eli, '' Trigonometric Delights '', Princeton Univ. Press. (1998). Reprint edition (February 25, 2002): ISBN 0-691-09541-8.

Needham, Tristan, "Preface" " to '' Visual Complex Analysis ''. Oxford University Press, (1999). ISBN 0-19-853446-9.

O'Connor, J.J., and E.F. Robertson, "Trigonometric functions" , '' MacTutor History Of Mathematics Archive ''. (1996).

O'Connor, J.J., and E.F. Robertson, "Madhava of Sangamagramma" , '' MacTutor History Of Mathematics Archive ''. (2000).

Pearce, Ian G., "Madhava of Sangamagramma" , '' MacTutor History Of Mathematics Archive ''. (2002).

Weisstein, Eric W., "Tangent" from '' MathWorld '', accessed 21 January 2006 .

SEE ALSO

Generating Trigonometric Tables

Hyperbolic Function

Pythagorean Theorem

Direction Cosines

Table Of Newtonian Series

Proofs Of Trigonometric Identities

Euler's Formula

All Students Take Calculus (A mnemonic for recalling the signs of trigonometric functions in a particular quadrant of a Cartesian plane)

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