त्रिकोणमितीय सर्वसमिकाओं की सूची

त्रिकोणमितीय सर्वसमिकाएँ, चरों के त्रिकोणमितीय फलनों के रूप में व्यक्त समतायें होती हैं।

कोण θ के सभी त्रिकोणमितीय फलनों को ज्यामितीय रूप से केन्द्रित एक इकाई वृत्त  O के रूप में निर्मित किया जा सकता है

The unit circle

प्रयुक्त संकेतों (Notation) के बारे में

To avoid the confusion caused by the ambiguity of sin⁻¹(x), the reciprocals and inverses of trigonometric functions are often displayed as in this table. In representing the cosecant function, the longer form 'cosec' is sometimes used in place of 'csc'.

Function		Inverse function		Reciprocal		Inverse reciprocal
sine	sin	arcsine	arcsin	cosecant	csc	arccosecant	arccsc
cosine	cos	arccosine	arccos	secant	sec	arcsecant	arcsec
tangent	tan	arctangent	arctan	cotangent	cot	arccotangent	arccot

Different angular measures can be appropriate in different situations. This table shows some of the more common systems. Radians is the default angular measure and is the one you use if you use the exponential definitions. All angular measures are unitless.

Degrees	30	45	60	90	120	180	270	360
Radians	${\displaystyle \pi /6}$	${\displaystyle \pi /4}$	${\displaystyle \pi /3}$	${\displaystyle \pi /2}$	${\displaystyle 2\pi /3}$	${\displaystyle \pi }$	${\displaystyle 3\pi /2}$	${\displaystyle 2\pi }$
Grads	33 ⅓	50	66 ⅔	100	133 ⅓	200	300	400

मौलिक सम्बन्ध

Pythagorean trigonometric identity	${\displaystyle \sin ^{2}\theta +\cos ^{2}\theta =1\,}$[1]
Ratio identity	${\displaystyle \tan \theta ={\frac {\sin \theta }{\cos \theta }}}$[1]

From the two identities above, the following table can be extrapolated. Note however that these conversion equations may not provide the correct sign (+ or −). For example, if sin θ = 1/2, the conversion in the table indicates that ${\displaystyle \scriptstyle \cos \theta \,=\,{\sqrt {1-\sin ^{2}\theta }}={\sqrt {3}}/2}$, though it is possible that ${\displaystyle \scriptstyle \cos \theta \,=\,-{\sqrt {3}}/2}$. More information would be needed about which quadrant θ lies in to determine a single, exact answer.

TRIGNOMETRY IMPORTANT FORMULAS

T. R.	sin	cos	tan	cosec	sec	cot
${\displaystyle \sin \theta =}$	${\displaystyle \sin \theta \ }$	${\displaystyle {\sqrt {1-\cos ^{2}\theta }}}$	${\displaystyle {\frac {\tan \theta }{\sqrt {1+\tan ^{2}\theta }}}}$	${\displaystyle {\frac {1}{\operatorname {cosec} \theta }}}$	${\displaystyle {\frac {\sqrt {\sec ^{2}\theta -1}}{\sec \theta }}}$	${\displaystyle {\frac {1}{\sqrt {1+\cot ^{2}\theta }}}}$
${\displaystyle \cos \theta =}$	${\displaystyle {\sqrt {1-\sin ^{2}\theta }}}$	${\displaystyle \cos \theta \ }$	${\displaystyle {\frac {1}{\sqrt {1+\tan ^{2}\theta }}}}$	${\displaystyle {\frac {\sqrt {\operatorname {cosec} ^{2}\theta -1}}{\operatorname {cosec} \theta }}}$	${\displaystyle {\frac {1}{\sec \theta }}}$	${\displaystyle {\frac {\cot \theta }{\sqrt {1+\cot ^{2}\theta }}}}$
${\displaystyle \tan \theta =}$	${\displaystyle {\frac {\sin \theta }{\sqrt {1-\sin ^{2}\theta }}}}$	${\displaystyle {\frac {\sqrt {1-\cos ^{2}\theta }}{\cos \theta }}}$	${\displaystyle \tan \theta \ }$	${\displaystyle {\frac {1}{\sqrt {\csc ^{2}\theta -1}}}}$	${\displaystyle {\sqrt {\sec ^{2}\theta -1}}}$	${\displaystyle {\frac {1}{\cot \theta }}}$
${\displaystyle \csc \theta =}$	${\displaystyle {1 \over \sin \theta }}$	${\displaystyle {1 \over {\sqrt {1-\cos ^{2}\theta }}}}$	${\displaystyle {{\sqrt {1+\tan ^{2}\theta }} \over \tan \theta }}$	${\displaystyle \csc \theta \ }$	${\displaystyle {\sec \theta \over {\sqrt {\sec ^{2}\theta -1}}}}$	${\displaystyle {\sqrt {1+\cot ^{2}\theta }}}$
${\displaystyle \sec \theta =}$	${\displaystyle {1 \over {\sqrt {1-\sin ^{2}\theta }}}}$	${\displaystyle {1 \over \cos \theta }}$	${\displaystyle {\sqrt {1+\tan ^{2}\theta }}}$	${\displaystyle {\csc \theta \over {\sqrt {\csc ^{2}\theta -1}}}}$	${\displaystyle \sec \theta \ }$	${\displaystyle {{\sqrt {1+\cot ^{2}\theta }} \over \cot \theta }}$
${\displaystyle \cot \theta =}$	${\displaystyle {{\sqrt {1-\sin ^{2}\theta }} \over \sin \theta }}$	${\displaystyle {\cos \theta \over {\sqrt {1-\cos ^{2}\theta }}}}$	${\displaystyle {1 \over \tan \theta }}$	${\displaystyle {\sqrt {\csc ^{2}\theta -1}}}$	${\displaystyle {1 \over {\sqrt {\sec ^{2}\theta -1}}}}$	${\displaystyle \cot \theta \ }$

ऐतिहासिक पृष्टभूमि

Rarely used today, the versine, coversine, haversine, and exsecant have been defined as below and used in navigation, for example the haversine formula was used to calculate the distance between two points on a sphere.

नाम	मान
${\displaystyle {\textrm {versin}}\,\theta }$	${\displaystyle 1-\cos \theta \,}$
${\displaystyle {\textrm {coversin}}\,\theta }$	${\displaystyle 1-\sin \theta \,}$
${\displaystyle {\textrm {haversin}}\,\theta }$	${\displaystyle {\tfrac {1}{2}}{\textrm {versin}}\theta \,}$
${\displaystyle {\textrm {exsec}}\,\theta \,}$	${\displaystyle \sec \theta -1\,}$

सममिति, अनतर एवं आवर्तन (Symmetry, shifts, and periodicity)

इकाई वृत्त (unit circle) का परीक्षण करके हम त्रिकोणमित्तीय फलनों के निम्नलिखित गुणों को स्थापित कर सकते हैं:

सममिति

जब त्रिकोणमितीय फलन ${\displaystyle \theta }$ के कुछ मानों से परिलक्षित होते हैं, तो परिणाम अक्सर अन्य त्रिकोणमितीय फलनों में से एक होता है। इससे निम्नलिखित सर्वसमिकाए प्राप्त होती है:

Reflected in ${\displaystyle \theta =0}$	Reflected in ${\displaystyle \theta =\pi /2}$	Reflected in ${\displaystyle \theta =\pi }$
${\displaystyle {\begin{aligned}\sin(0-\theta )&=-\sin \theta \\\cos(0-\theta )&=+\cos \theta \\\tan(0-\theta )&=-\tan \theta \\\csc(0-\theta )&=-\csc \theta \\\sec(0-\theta )&=+\sec \theta \\\cot(0-\theta )&=-\cot \theta \end{aligned}}}$	${\displaystyle {\begin{aligned}\sin({\tfrac {\pi }{2}}-\theta )&=+\cos \theta \\\cos({\tfrac {\pi }{2}}-\theta )&=+\sin \theta \\\tan({\tfrac {\pi }{2}}-\theta )&=+\cot \theta \\\csc({\tfrac {\pi }{2}}-\theta )&=+\sec \theta \\\sec({\tfrac {\pi }{2}}-\theta )&=+\csc \theta \\\cot({\tfrac {\pi }{2}}-\theta )&=+\tan \theta \end{aligned}}}$	${\displaystyle {\begin{aligned}\sin(\pi -\theta )&=+\sin \theta \\\cos(\pi -\theta )&=-\cos \theta \\\tan(\pi -\theta )&=-\tan \theta \\\csc(\pi -\theta )&=+\csc \theta \\\sec(\pi -\theta )&=-\sec \theta \\\cot(\pi -\theta )&=-\cot \theta \\\end{aligned}}}$

आवर्तता (periodicity)

By shifting the function round by certain angles, it is often possible to find different trigonometric functions that express the result more simply. Some examples of this are given shown by shifting functions round by π/2, π and 2π radians. Because the periods of these functions are either π or 2π, there are cases where the new function is exactly the same as the old function without the shift.

Shift by π/2	Shift by π Period for tan and cot	Shift by 2π Period for sin, cos, csc and sec
${\displaystyle {\begin{aligned}\sin(\theta +{\tfrac {\pi }{2}})&=+\cos \theta \\\cos(\theta +{\tfrac {\pi }{2}})&=-\sin \theta \\\tan(\theta +{\tfrac {\pi }{2}})&=-\cot \theta \\\csc(\theta +{\tfrac {\pi }{2}})&=+\sec \theta \\\sec(\theta +{\tfrac {\pi }{2}})&=-\csc \theta \\\cot(\theta +{\tfrac {\pi }{2}})&=-\tan \theta \end{aligned}}}$	${\displaystyle {\begin{aligned}\sin(\theta +\pi )&=-\sin \theta \\\cos(\theta +\pi )&=-\cos \theta \\\tan(\theta +\pi )&=+\tan \theta \\\csc(\theta +\pi )&=-\csc \theta \\\sec(\theta +\pi )&=-\sec \theta \\\cot(\theta +\pi )&=+\cot \theta \\\end{aligned}}}$	${\displaystyle {\begin{aligned}\sin(\theta +2\pi )&=+\sin \theta \\\cos(\theta +2\pi )&=+\cos \theta \\\tan(\theta +2\pi )&=+\tan \theta \\\csc(\theta +2\pi )&=+\csc \theta \\\sec(\theta +2\pi )&=+\sec \theta \\\cot(\theta +2\pi )&=+\cot \theta \end{aligned}}}$

कोणों के योग और अन्तर की सर्वसमिकाएँ (Angle sum and difference identities)

इनको योग एवं अन्तर के सूत्र भी कहते हैं। इनको यूलर के सूत्र की सहायता से बडी आसानी से सिद्ध किया जा सकता है।

Sine	${\displaystyle \sin(\theta \pm \phi )=\sin \theta \cos \phi \pm \cos \theta \sin \phi \,}$[2]	Note: From Plus-minus sign#Minus-plus sign. ${\displaystyle {\begin{aligned}x\pm y=a\pm b&\Rightarrow \ x+y=a+b\\&{\mbox{and}}\ x-y=a-b\end{aligned}}}$ ${\displaystyle {\begin{aligned}x\pm y=a\mp b&\Rightarrow \ x+y=a-b\\&{\mbox{and}}\ x-y=a+b\end{aligned}}}$
Cosine	${\displaystyle \cos(\theta \pm \phi )=\cos \theta \cos \phi \mp \sin \theta \sin \phi \,}$[2]
Tangent	${\displaystyle \tan(\theta \pm \phi )={\frac {\tan \theta \pm \tan \phi }{1\mp \tan \theta \tan \phi }}}$[2]

अनन्त पदों के योग की ज्या एवं कोज्या

(Sines and cosines of sums of infinitely many terms)

${\displaystyle \sin \left(\sum _{i=1}^{\infty }\theta _{i}\right)=\sum _{\mathrm {odd} \ k\geq 1}(-1)^{(k-1)/2}\sum _{|A|=k}\left(\prod _{i\in A}\sin \theta _{i}\prod _{i\not \in A}\cos \theta _{i}\right)}$

${\displaystyle \cos \left(\sum _{i=1}^{\infty }\theta _{i}\right)=\sum _{\mathrm {even} \ k\geq 0}~(-1)^{k/2}~~\sum _{|A|=k}\left(\prod _{i\in A}\sin \theta _{i}\prod _{i\not \in A}\cos \theta _{i}\right)}$

where "|A| = k" means the index A runs through the set of all subsets of size k of the set { 1, 2, 3, ... }.

In these two identities an asymmetry appears that is not seen in the case of sums of finitely many terms: in each product, there are only finitely many sine factors and cofinitely many cosine factors.

If only finitely many of the terms θ_i are nonzero, then only finitely many of the terms on the right side will be nonzero because sine factors will vanish, and in each term, all but finitely many of the cosine factors will be unity.

अनेक किन्तु सीमित पदों के योग की स्पर्शज्या

(Tangents of sums of finitely many terms)

Let x_i = tan(θ_i ), for i = 1, ..., n. Let e_k be the kth-degree elementary symmetric polynomial in the variables x_i, i = 1, ..., n, k = 0, ..., n. Then

${\displaystyle \tan(\theta _{1}+\cdots +\theta _{n})={\frac {e_{1}-e_{3}+e_{5}-\cdots }{e_{0}-e_{2}+e_{4}-\cdots }},}$

the number of terms depending on n.

उदाहरण्के लिये,

${\displaystyle {\begin{aligned}\tan(\theta _{1}+\theta _{2}+\theta _{3})&{}={\frac {e_{1}-e_{3}}{e_{0}-e_{2}}}={\frac {(x_{1}+x_{2}+x_{3})\ -\ (x_{1}x_{2}x_{3})}{1\ -\ (x_{1}x_{2}+x_{1}x_{3}+x_{2}x_{3})}},\\\\\tan(\theta _{1}+\theta _{2}+\theta _{3}+\theta _{4})&{}={\frac {e_{1}-e_{3}}{e_{0}-e_{2}+e_{4}}}\\\\&{}={\frac {(x_{1}+x_{2}+x_{3}+x_{4})\ -\ (x_{1}x_{2}x_{3}+x_{1}x_{2}x_{4}+x_{1}x_{3}x_{4}+x_{2}x_{3}x_{4})}{1\ -\ (x_{1}x_{2}+x_{1}x_{3}+x_{1}x_{4}+x_{2}x_{3}+x_{2}x_{4}+x_{3}x_{4})\ +\ (x_{1}x_{2}x_{3}x_{4})}},\end{aligned}}}$

आदि आदि। इसकी सामान्य स्थिति (general case) गणितीय निगमन (mathematical induction) की सहायता से सिद्ध किया जा सकता है।

कोणों के गुणज के त्रिकोणमित्तीय अनुपात

(Multiple-angle formulae)

${\displaystyle \cos n\theta =T_{n}(\cos \theta )\,}$[3]	Tn nवाँ चेविशेव बहुपद (Chebyshev polynomial) है।
${\displaystyle \sin ^{2}n\theta =S_{n}(\sin ^{2}\theta )\,}$	Sn nवाँ spread polynomial है।
${\displaystyle \cos n\theta +i\sin n\theta =(\cos(\theta )+i\sin(\theta ))^{n}\,}$	द मायवर का सूत्र (de Moivre's formula), ${\displaystyle i}$ काल्पनिक इकाई (Imaginary unit) है।

${\displaystyle 1+2\cos(x)+2\cos(2x)+2\cos(3x)+\cdots +2\cos(nx)={\frac {\sin \left(\left(n+{\frac {1}{2}}\right)x\right)}{\sin(x/2)}}.}$

(x के इस फलन (फंक्शन) को डिरिचलेट कर्नेल (Dirichlet kernel) कहते हैं।

दोगुना, तिगुना एवं आधे कोण के सूत्र

(Double-, triple-, and half-angle formulae)

इन्हें योग एवं अन्तर की सर्वसमिकाओं की सहायता से या गुणज-कोण की सर्वसमिकाओं की सहायता से प्रदर्शित (सिद्ध) किया जा सकता है।

Double-angle formulae[4]
पार्स नहीं कर पाये (सिन्टैक्स त्रुटि): {\displaystyle \begin|style="vertical-align:top"|<math>\begin{align} \sin 2\theta &= 2 \sin \theta \cos \theta \ \\ &= \frac{2 \tan \theta} {1 + \tan^2 \theta} \end{align}}	${\displaystyle {\begin{aligned}\cos 2\theta &=\cos ^{2}\theta -\sin ^{2}\theta \\&=2\cos ^{2}\theta -1\\&=1-2\sin ^{2}\theta \\&={\frac {1-\tan ^{2}\theta }{1+\tan ^{2}\theta }}\end{aligned}}}$	${\displaystyle \tan 2\theta ={\frac {2\tan \theta }{1-\tan ^{2}\theta }}\,}$	${\displaystyle \cot 2\theta ={\frac {\cot \theta -\tan \theta }{2}}\,}$
Triple-angle formulae[3]
${\displaystyle \sin 3\theta =3\sin \theta -4\sin ^{3}\theta \,}$	${\displaystyle \cos 3\theta =4\cos ^{3}\theta -3\cos \theta \,}$	${\displaystyle \tan 3\theta ={\frac {3\tan \theta -\tan ^{3}\theta }{1-3\tan ^{2}\theta }}}$
Half-angle formulae[5]
${\displaystyle \sin {\tfrac {\theta }{2}}=\pm \,{\sqrt {\frac {1-\cos \theta }{2}}}}$	${\displaystyle \cos {\tfrac {\theta }{2}}=\pm \,{\sqrt {\frac {1+\cos \theta }{2}}}}$	${\displaystyle {\begin{aligned}\tan {\tfrac {\theta }{2}}&=\csc \theta -\cot \theta \\&=\pm \,{\sqrt {1-\cos \theta \over 1+\cos \theta }}\\&={\frac {\sin \theta }{1+\cos \theta }}\\&={\frac {1-\cos \theta }{\sin \theta }}\end{aligned}}}$	${\displaystyle \cot {\tfrac {\theta }{2}}=\csc \theta +\cot \theta }$

See also Tangent half-angle formula.

यूलर का अनन्त गुणनफल

(Euler's infinite product )

${\displaystyle \cos \left({\theta \over 2}\right)\cdot \cos \left({\theta \over 4}\right)\cdot \cos \left({\theta \over 8}\right)\cdots =\prod _{n=1}^{\infty }\cos \left({\theta \over 2^{n}}\right)={\sin(\theta ) \over \theta }.}$

घातांक कम करने का सूत्र

(Power-reduction formulae)

इन्हें कोज्या द्वि-गुण कोण सूत्र के द्वीतीय एवं तृतीय रूप का प्रयोग करके प्राप्त किया जा सकता है।

Sine	${\displaystyle \sin ^{2}\theta ={\frac {1-\cos 2\theta }{2}}}$	${\displaystyle \sin ^{3}\theta ={\frac {3\sin \theta -\sin 3\theta }{4}}}$
Cosine	${\displaystyle \cos ^{2}\theta ={\frac {1+\cos 2\theta }{2}}}$	${\displaystyle \cos ^{3}\theta ={\frac {3\cos \theta +\cos 3\theta }{4}}}$
Other	${\displaystyle \sin ^{2}\theta \cos ^{2}\theta ={\frac {1-\cos 4\theta }{8}}}$	${\displaystyle \sin ^{3}\theta \cos ^{3}\theta ={\frac {\sin ^{3}2\theta }{8}}}$

गुणनफल-से-योग तथा योग-से-गुणनफल की सर्वसमिकाएँ

(Product-to-sum and sum-to-product identities)

Product-to-sum ${\displaystyle \cos \theta \cos \phi ={\cos(\theta -\phi )+\cos(\theta +\phi ) \over 2}}$ ${\displaystyle \sin \theta \sin \phi ={\cos(\theta -\phi )-\cos(\theta +\phi ) \over 2}}$ ${\displaystyle \sin \theta \cos \phi ={\sin(\theta +\phi )+\sin(\theta -\phi ) \over 2}}$

Sum-to-product ${\displaystyle \sin \theta +\sin \phi =2\sin \left({\frac {\theta +\phi }{2}}\right)\cos \left({\frac {\theta -\phi }{2}}\right)}$ ${\displaystyle \cos \theta +\cos \phi =2\cos \left({\frac {\theta +\phi }{2}}\right)\cos \left({\frac {\theta -\phi }{2}}\right)}$ ${\displaystyle \cos \theta -\cos \phi =-2\sin \left({\theta +\phi \over 2}\right)\sin \left({\theta -\phi \over 2}\right)}$ ${\displaystyle \sin \theta -\sin \phi =2\cos \left({\theta +\phi \over 2}\right)\sin \left({\theta -\phi \over 2}\right)\;}$

अन्य सम्बन्धित सर्वसमिकाएँ

If x, y, and z are the three angles of any triangle, or in other words

${\displaystyle {\mbox{if }}x+y+z=\pi ={\mbox{half circle,}}\,}$

${\displaystyle {\mbox{then }}\tan(x)+\tan(y)+\tan(z)=\tan(x)\tan(y)\tan(z).\,}$

(If any of x, y, z is a right angle, one should take both sides to be ∞. This is neither +∞ nor −∞; for present purposes it makes sense to add just one point at infinity to the real line, that is approached by tan(θ) as tan(θ) either increases through positive values or decreases through negative values. This is a one-point compactification of the real line.)

${\displaystyle {\mbox{If }}x+y+z=\pi ={\mbox{half circle,}}\,}$

${\displaystyle {\mbox{then }}\sin(2x)+\sin(2y)+\sin(2z)=4\sin(x)\sin(y)\sin(z).\,}$

टोलेमी का प्रमेय (Ptolemy's theorem)

${\displaystyle {\mbox{If }}w+x+y+z=\pi ={\mbox{half circle,}}\,}$

${\displaystyle {\begin{aligned}{\mbox{then }}&\sin(w+x)\sin(x+y)\\&{}=\sin(x+y)\sin(y+z)\\&{}=\sin(y+z)\sin(z+w)\\&{}=\sin(z+w)\sin(w+x)=\sin(w)\sin(y)+\sin(x)\sin(z).\end{aligned}}}$

(The first three equalities are trivial; the fourth is the substance of this identity.) Essentially this is Ptolemy's theorem adapted to the language of trigonometry.

Linear combinations

For some purposes it is important to know that any linear combination of sine waves of the same period but different phase shifts is also a sine wave with the same period, but a different phase shift. In the case of a linear combination of a sine and cosine wave, we have

${\displaystyle a\sin x+b\cos x={\sqrt {a^{2}+b^{2}}}\cdot \sin(x+\varphi )\,}$

where

${\displaystyle \varphi =\arctan \left({\frac {b}{a}}\right)}$

More generally, for an arbitrary phase shift, we have

${\displaystyle a\sin x+b\sin(x+\alpha )=c\sin(x+\beta )\,}$

where

${\displaystyle c={\sqrt {a^{2}+b^{2}+2ab\cos \alpha }},}$

and

${\displaystyle \beta ={\rm {arctan}}\left({\frac {b\sin \alpha }{a+b\cos \alpha }}\right).}$

• note: arcsin, arccos, arctan are all inverses.

Other sums of trigonometric functions

Sum of sines and cosines with arguments in arithmetic progression:

${\displaystyle \sin {\varphi }+\sin {(\varphi +\alpha )}+\sin {(\varphi +2\alpha )}+\cdots +\sin {(\varphi +n\alpha )}={\frac {\sin {\left({\frac {(n+1)\alpha }{2}}\right)}\cdot \sin {(\varphi +{\frac {n\alpha }{2}})}}{\sin {\frac {\alpha }{2}}}}.}$

${\displaystyle \cos {\varphi }+\cos {(\varphi +\alpha )}+\cos {(\varphi +2\alpha )}+\cdots +\cos {(\varphi +n\alpha )}={\frac {\sin {\left({\frac {(n+1)\alpha }{2}}\right)}\cdot \cos {(\varphi +{\frac {n\alpha }{2}})}}{\sin {\frac {\alpha }{2}}}}.}$

For any a and b:

${\displaystyle a\cos(x)+b\sin(x)={\sqrt {a^{2}+b^{2}}}\cos(x-\arctan(b,a))\;}$

where arctan(y, x) is the generalization of arctan(y/x) which covers the entire circular range (see also the account of this same identity in "symmetry, periodicity, and shifts" above for this generalization of arctan).

${\displaystyle \tan(x)+\sec(x)=\tan \left({x \over 2}+{\pi \over 4}\right).}$

The above identity is sometimes convenient to know when thinking about the Gudermanian function.

If x, y, and z are the three angles of any triangle, i.e. if x + y + z = π, then

${\displaystyle \cot(x)\cot(y)+\cot(y)\cot(z)+\cot(z)\cot(x)=1.\,}$

व्युत्क्रम त्रिकोणमित्तीय फलन

(Inverse trigonometric functions)

पार्स नहीं कर पाये (सिन्टैक्स त्रुटि): {\displaystyle \2arcsin(1/2)+\2arccos(-1/2)+3tan(-1) =\pi/2\;}

${\displaystyle \arctan(x)+\operatorname {arccot}(x)=\pi /2.\;}$

${\displaystyle \arctan(x)+\arctan(1/x)=\left\{{\begin{matrix}\pi /2,&{\mbox{if }}x>0\\-\pi /2,&{\mbox{if }}x<0\end{matrix}}\right.}$

Compositions of trig and inverse trig functions

Relation to the complex exponential function

${\displaystyle e^{ix}=\cos(x)+i\sin(x)\,}$ (Euler's formula),

${\displaystyle e^{-ix}=\cos(-x)+i\sin(-x)=\cos(x)-i\sin(x)\,}$

${\displaystyle e^{i\pi }=-1\,}$

${\displaystyle \cos(x)={\frac {e^{ix}+e^{-ix}}{2}}\;}$

${\displaystyle \sin(x)={\frac {e^{ix}-e^{-ix}}{2i}}\;}$

where i² = −1.

"cis"

साँचा:Split-section Occasionally one sees the notation

<math>\operatorname{cis}(x) = \cos(x) + i\sin(x),\,</math>

i.e. "cis" abbreviates "cos&nbsp;+&nbsp;i&nbsp;sin".

Though at first glance this notation is redundant, being equivalent to e^ix, its use is rooted in several advantages.

Convenience

This notation was more common in the post WWII era when typewriters were used to convey mathematical expressions. Superscripts are both offset vertically and smaller than 'cis' or 'exp'; hence, they can be problematic even for hand writing. For example e^ix² versus cis(x²) versus exp(ix²). For many readers, cis(x²) is the clearest, easiest to read of the three.

The cis notation is sometimes used to emphasize one method of viewing and dealing with a problem over another. The mathematics of trigonometry and exponentials are related but not exactly the same; exponential notation emphasizes the whole, whereas cis and cos&nbsp;+&nbsp;i&nbsp;sin notations emphasize the parts. This can be rhetorically useful to mathematicians and engineers when discussing this function, and further serve as a mnemonic (for cos&nbsp;+&nbsp;i&nbsp;sin).

The cis notation is convenient for math students whose knowledge of trigonometry and complex numbers permit this notation, but whose conceptual understanding doesn't yet permit the notation e^ix. As students learn concepts that build on prior knowledge, it is important not to force them into levels of math they are not yet prepared for.

Pedagogy

In some contexts, the cis notation may serve the pedagogical purpose of emphasizing that one has not yet proved that this is an exponential function. In doing trigonometry without complex numbers, one may prove the two identities

<math>\cos(x+y) = \cos(x)\cos(y) - \sin(x)\sin(y) = c_1 c_2 - s_1 s_2,\,</math>

<math>\sin(x+y) = \sin(x)\cos(y) + \cos(x)\sin(y) = s_1 c_2 + c_1 s_2.\,</math>

Similarly in treating multiplication of complex numbers (with no involvement of trigonometry), one may observe that the real and imaginary parts of the product of c₁&nbsp;+&nbsp;is₁ and c₂&nbsp;+&nbsp;is₂ are respectively

<math>c_1 c_2 - s_1 s_2,\,</math>

<math>s_1 c_2 + c_1 s_2.\,</math>

Thus one sees this same pattern arising in two disparate contexts:

• trigonometry without complex numbers, and
• complex numbers without trigonometry.

This coincidence can serve as a motivation for conjoining the two contexts and thereby discovering the trigonometric identity

<math>\operatorname{cis}(x+y) = \operatorname{cis}(x)\operatorname{cis}(y),\,</math>

and observing that this identity for cis of a sum is simpler than the identities for sin and cos of a sum. Having proved this identity, one can challenge the students to recall which familiar sort of function satisfies this same functional equation

<math>f(x+y) = f(x)f(y).\,</math>

The answer is exponential functions. That suggests that cis may be an exponential function

<math>\operatorname{cis}(x) = b^x.\,</math>

Then the question is: what is the base b? The definition of cis and the local behavior of sin and cos near zero suggest that

<math>\operatorname{cis}(0+dx) = \operatorname{cis}(0) + i\,dx,</math>

(where dx is an infinitesimal increment of x). Thus the rate of change at 0 is i, so the base should be eⁱ. Thus if this is an exponential function, then it must be

<math>\operatorname{cis}(x) = e^{ix}.\,</math>