The study of angles and of the angular relationships of planar and three-dimensional figures is known as trigonometry. The trigonometric
functions (also called the circular functions )
comprising trigonometry are the cosecant , cosine , cotangent , secant , sine , and tangent . The inverses of these functions are denoted , , , , , and . Note that the notation here means inverse
function , not
to the power .
The trigonometric functions are most simply defined using the unit circle . Let
be an angle measured counterclockwise from the x -axis
along an arc of the circle . Then
is the horizontal coordinate
of the arc endpoint, and is the vertical component. The ratio is defined as . As a result of this definition, the trigonometric
functions are periodic with period , so
(1)
where
is an integer and func is a trigonometric function.
A right triangle has three sides, which can be uniquely identified as the hypotenuse , adjacent to
a given angle ,
or opposite .
A helpful mnemonic for remembering the definitions of the trigonometric functions
is then given by "oh, ah, o-a," "Soh, Cah, Toa," or "SOHCAHTOA ", i.e., sine equals opposite over hypotenuse,
cosine equals adjacent over hypotenuse, and tangent equals opposite over adjacent,
Another mnemonic probably more common in Great Britain than the United States is "Tommy On A Ship Of His Caught A Herring."
From the Pythagorean theorem ,
(5)
It is therefore also true that
(6)
and
(7)
The trigonometric functions can be defined algebraically in terms of complex exponentials (i.e., using the Euler
formula ) as
Hybrid trigonometric product/sum formulas are
Osborn's rule gives a prescription for converting trigonometric identities to analogous identities for hyperbolic
functions .
For imaginary arguments,
For complex arguments,
For the absolute square of complex arguments ,
The complex modulus also satisfies the curious
identity
(29)
The only functions satisfying identities of this form,
(30)
are ,
, and (Robinson 1957).
See also Cosecant ,
Cosine ,
Cotangent ,
Double-Angle
Formulas ,
Euclidean Number ,
Half-Angle
Formulas ,
Inverse Cosecant ,
Inverse
Cosine ,
Inverse Cotangent ,
Inverse
Secant ,
Inverse Sine ,
Inverse
Tangent ,
Inverse Trigonometric
Functions ,
Multiple-Angle Formulas ,
Osborn's Rule ,
Polygon ,
Prosthaphaeresis Formulas ,
Secant ,
Sine ,
SOHCAHTOA ,
Tangent ,
Trigonometric Addition Formulas ,
Trigonometry Angles ,
Trigonometric
Functions ,
Trigonometric Power Formulas ,
Trigonometric Series Formulas ,
Unit Circle ,
Werner Formulas Explore this topic
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References Abramowitz, M. and Stegun, I. A. (Eds.). "Circular Functions." §4.3 in Handbook
of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, 9th printing.
New York: Dover, pp. 71-79, 1972. Bahm, L. B. The
New Trigonometry on Your Own. Patterson, NJ: Littlefield, Adams & Co.,
1964. Beyer, W. H. "Trigonometry." CRC
Standard Mathematical Tables, 28th ed. Boca Raton, FL: CRC Press, pp. 134-152,
1987. Borchardt, W. G. and Perrott, A. D. A
New Trigonometry for Schools. London: G. Bell, 1930. Dixon, R.
"The Story of Sine and Cosine." §4.4 in Mathographics.
New York: Dover, pp. 102-106, 1991. Hobson, E. W. A
Treatise on Plane Trigonometry. London: Cambridge University Press, 1925. Kells,
L. M.; Kern, W. F.; and Bland, J. R. Plane
and Spherical Trigonometry. New York: McGraw-Hill, 1940. Maor,
E. Trigonometric
Delights. Princeton, NJ: Princeton University Press, 1998. Morrill,
W. K. Plane
Trigonometry, rev. ed. Dubuque, IA: Wm. C. Brown, 1964. Robinson,
R. M. "A Curious Mathematical Identity." Amer. Math. Monthly 64 ,
83-85, 1957. Siddons, A. W. and Hughes, R. T. Trigonometry,
Part I. London: Cambridge University Press, 1929a. Siddons, A. W.
and Hughes, R. T. Trigonometry, Part II. London: Cambridge University
Press, 1929b. Siddons, A. W. and Hughes, R. T. Trigonometry,
Part III. London: Cambridge University Press, 1929c. Siddons,
A. W. and Hughes, R. T. Trigonometry, Part IV. London: Cambridge
University Press, 1929d. Sloane, N. J. A. Sequence A003401 /M0505
in "The On-Line Encyclopedia of Integer Sequences." Thompson,
J. E. Trigonometry
for the Practical Man. Princeton, NJ: Van Nostrand, 1946. Yates,
R. C. "Trigonometric Functions." A
Handbook on Curves and Their Properties. Ann Arbor, MI: J. W. Edwards,
pp. 225-232, 1952. Weisstein, E. W. "Books about Trigonometry."
http://www.ericweisstein.com/encyclopedias/books/Trigonometry.html . Zill,
D. G. and Dewar, J. M. Trigonometry,
2nd ed. New York: McGraw-Hill 1990. Referenced on Wolfram|Alpha Trigonometry
Cite this as:
Weisstein, Eric W. "Trigonometry." From
MathWorld --A Wolfram Web Resource. https://mathworld.wolfram.com/Trigonometry.html
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