# Complex numbers techniques !

This note is for complex numbers lovers.

I have very keen interest in complex numbers and as many of you also are mad about using Complex Numbers in Various Section of Maths! Since It is Modern Mathematics Technique , And It Highly reduce our calculation .

So Here in this Note I want to share Some *Complex Number Techniques That I learnt till Now . And It is Humble Request to Post Related Complex number Techniques which You Had Learnt or Created at your Own ! So That Our Brilliant Community Learn from it.*

A few of them are :

$\bullet$ 1 ) - Find Summation of Series :

$C\quad =\quad \cos { \theta } +\cos { 2\theta } \quad +\cos { 3\theta } +\quad .\quad .\quad .\quad .\quad .\quad .\quad +\quad \cos { (n\theta ) } \\ \\ S\quad =\quad \sin { \theta } +\quad \sin { 2\theta } \quad +\sin { 3\theta } +\quad .\quad .\quad .\quad .\quad .\quad .\quad \quad +\quad \sin { (n\theta ) }$.

By Using Euler's and Converting into :

$C\quad +\quad iS\quad =\quad { e }^{ i\theta }\quad +\quad { e }^{ i2\theta }\quad +\quad { e }^{ i3\theta }\quad +\quad .\quad .\quad .\quad .\quad .\quad .\quad .\quad { e }^{ i(n\theta ) }\quad \quad \quad \quad (\quad G.P\quad )\\ \\ \\ C\quad +\quad iS\quad =\frac { { e }^{ i\theta }({ e }^{ i(n\theta ) }\quad -\quad 1) }{ { e }^{ i\theta }\quad -\quad 1 }$.

And Then Separate real and imaginary Part And Then Compare !

$\bullet$ 2 )- Find Integrals ${ I }_{ 1 }\quad =\int { { e }^{ ax }\cos { bx } } dx\quad \quad \& \quad { I }_{ 2 }\quad =\int { { e }^{ ax }\sin { bx } } dx\quad$.

By Considering ${ I }_{ 1 }\quad +\quad i{ \cdot I }_{ 1 }\quad =\int { { e }^{ ax }(\cos { bx } +i\cdot \sin { bx) } } dx\quad =\quad \int { { e }^{ (a+ib)x } } dx\quad \\ \\ { I }_{ 1 }\quad +\quad i{ \cdot I }_{ 1 }\quad =\quad \cfrac { { e }^{ (a+ib)x } }{ a+ib }$.

And then Separate Real and imaginary Part and Then Compare !

$\bullet$ 3 )- TPT : $\sin { \cfrac { \pi }{ 7 } } \cdot \sin { \cfrac { 2\pi }{ 7 } } \cdot \sin { \cfrac { 3\pi }{ 7 } } \quad =\quad \cfrac { \sqrt { 7 } }{ 8 }$.

By Considering 7th Root's of unity :

$1\quad ,\quad { \alpha }_{ 1 }\quad ,\quad { \alpha }_{ 2 }\quad ,\quad \quad .\quad .\quad .\quad .\quad .\quad ,\quad { \alpha }_{ 6 }\quad \quad (\because \quad { \alpha }_{ k }\quad =\quad { e }^{ \cfrac { 2k\pi i }{ 7 } }\quad )$.

And Using Property n'th roots of unity ( Here n = 7 ):

$\left| (1\quad -\quad { \alpha }_{ 1 })(1\quad -\quad { \alpha }_{ 2 })(1\quad -\quad { \alpha }_{ 3 })\quad .\quad .\quad .\quad .\quad .\quad (1\quad -\quad { \alpha }_{ 6 }) \right| \quad =\quad \left| \quad 7\quad \right|$.

And Now : $1\quad -\quad { \alpha }_{ K }\quad =\quad 1\quad -\quad (\cos { \cfrac { 2\pi k }{ 7 } } \quad +\quad i\cdot \sin { \cfrac { 2\pi k }{ 7 } } )\\ \\ 1\quad -\quad { \alpha }_{ K }\quad =\quad 2\sin { \cfrac { \pi k }{ 7 } } (\cos { \cfrac { \pi k }{ 7 } } \quad -\quad i\cdot \sin { \cfrac { \pi k }{ 7 } } )\\ \\ \left| 1\quad -\quad { \alpha }_{ K } \right| =\quad 2\sin { \cfrac { \pi k }{ 7 } } \quad \quad \quad \quad \quad (II)\\ \\ \left| (1\quad -\quad { \alpha }_{ 1 })(1\quad -\quad { \alpha }_{ 2 })(1\quad -\quad { \alpha }_{ 3 })\quad .\quad .\quad .\quad .\quad .\quad (1\quad -\quad { \alpha }_{ 6 }) \right| =\quad 7\\ \\ { 2 }^{ 6 }\times { (\sin { \cfrac { \pi }{ 7 } } \cdot \sin { \cfrac { 2\pi }{ 7 } } \cdot \sin { \cfrac { 3\pi }{ 7 } } ) }^{ 2 }\quad =\quad 7\\ \\ \sin { \cfrac { \pi }{ 7 } } \cdot \sin { \cfrac { 2\pi }{ 7 } } \cdot \sin { \cfrac { 3\pi }{ 7 } } \quad =\quad \cfrac { \sqrt { 7 } }{ 8 }$.

I'am Done ! Now It's Your Turn .

Enjoy Complex !! :) :)

Re-share This More And More So that it reaches to every complex numbers Lovers , So that we can learn new Techniques !

Note by Deepanshu Gupta
5 years, 6 months ago

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Expanding $cos^{7}x$ in terms a series of cosines of multiples of x.

$y = cosx + isinx$

$y^{n} + \dfrac{1}{y^{n}} = 2cosnx$ , ( Since $y={ e }^{ ix }$ )

$y + \dfrac{1}{y} = 2cosx$

$cos^{7}x = ( y + \dfrac{1}{y})^{7}$

$= y^{7} + 7y^{5} + 21y^{3} + 35y + 35\dfrac{1}{y} + 21\dfrac{1}{y^{3}} + 7\dfrac{1}{y^{5}} + \dfrac{1}{y^{7}}$

$= ( y^{7} + \dfrac{1}{y^{7}}) + 7(y^{5} + \dfrac{1}{y^{5}}) + 21(y^{3} + \dfrac{1}{y^{3}}) + 35(y + \dfrac{1}{y})$

$= 2cos7x + 7.2cos5x + 21.2cos3x + 35.2cosx$

$cos^{7}x = \dfrac{1}{64}(cos7x + 7cos5x + 21cos3x + 35cosx)$

- 5 years, 5 months ago

$\lim _{ n\rightarrow \infty }{ \prod _{ r=1 }^{ n }{ { (Very\quad Nice) }^{ r } } }$.

- 5 years, 5 months ago

Awsome @megh choksi thanks for sharing

- 5 years, 5 months ago

Well, there are a lot of applications of Euler's theorem.

For example :

1) $\displaystyle \sum_{r=0}^{n} {n \choose r} \cos (r x) = \Re \bigg(\sum_{r=0}^{n} {n \choose r} e^{i r x}\bigg)$

2) $\displaystyle \sum_{r=0}^{\infty} \dfrac{\cos rx}{2^r} = \Re \bigg(\sum_{r=0}^{\infty} \bigg(\dfrac {e^{ix}}{2} \bigg)^r \bigg)$

3) Expressing $\sin rx$ and $\cos rx$ in terms of $\sin x$ and $\cos x$

- 5 years, 6 months ago

Did a silly thing , sorry i am posting it here

- 5 years, 5 months ago

Lol ! :)

You r creative :) :)

- 5 years, 5 months ago

@Deepanshu Gupta awsone post............i need your help how to solve following problems:-

(1)..Let $z_1,z_2,z_3$ be three complex numbers such that:- $|z_1|=|z_2|=|z_3|=|\frac{1}{z_1}+\frac{1}{z_2}+\frac{1}{z_3}|=1$ Find the value of $z_1z_2z_3$

(2)let z be a complex number such that:- $|z+\frac{4}{z}|=2$ Find maximum value of |z|

(3)let z be a complex number then find maximum value of |z|+|z-1|

(4)if $z^2+z+1=0$ where z is a complex number then find tje value of:- $(z+\frac{1}{z})^2 + (z^2+\frac{1}{z^2})^2+...........+(z^6+\frac{1}{z^6})^2$

Please help me how to solve these problems...i know it is off topic i am sorry for that......i am a beginer so please post solutions...... also sorry to desturb you

- 5 years, 6 months ago

Hints:

Sol 4 ) - Note Roots of quadratic are $\quad \omega \quad ,\quad { \omega }^{ 2 }\quad \longrightarrow \quad Cube\quad root\quad of\quad unity$

And Use Following Properties To get Answer :

$\quad \omega \quad +\quad { \omega }^{ 2 }\quad +\quad { \omega }^{ 3 }\quad =\quad 0\quad \quad \quad (\therefore \quad \omega \quad +\quad { \omega }^{ 2 }\quad =\quad -1\quad )\\ \quad \quad \quad \quad \quad \& \quad \quad \omega \quad =\quad \cfrac { 1 }{ { \omega }^{ 2 } } \quad$.

Sol 2)- Let $\left| z \right| \quad =\quad r$.

use Triangle inequality :

$\\ \left| r-\cfrac { 4 }{ r } \right| \quad \le \quad \left| z\quad +\quad \cfrac { 4 }{ z } \right| \quad \le \quad r\quad +\quad \cfrac { 4 }{ r } \\ \\ \left| r-\cfrac { 4 }{ r } \right| \quad \le \quad 2\quad \quad \quad .\quad .\quad .(I)\\ \quad r\quad +\quad \cfrac { 4 }{ r } \quad \ge \quad 2\quad \quad \quad \quad (Useless\quad \because \quad True\quad \forall \quad r\quad \quad by\quad AM-GM)$.

Now Solve $I$ equation and get required maximum value of "r" .

I will Post rest of two later !

- 5 years, 6 months ago

3) I think there is something wrong with it as when $a,b$ increases where $z=a+ib$ $a,b$ belongs to R, then |z| increases and also |z-1| so it will grow to infinity.

So the answer is infinity or a typo.

- 5 years, 5 months ago

Just one word; Brilliant! :)

- 4 years, 7 months ago