Complex exponential function
$begingroup$
Just a basic question. I have $e^{-i9pi/4}$, and I'm struggling to understand why this is equal to $e^{-ipi/4}$.
Thanks in advance for any help.
complex-numbers
$endgroup$
add a comment |
$begingroup$
Just a basic question. I have $e^{-i9pi/4}$, and I'm struggling to understand why this is equal to $e^{-ipi/4}$.
Thanks in advance for any help.
complex-numbers
$endgroup$
4
$begingroup$
Poles? $ $ $ $ $ $
$endgroup$
– Did
May 23 '11 at 14:10
add a comment |
$begingroup$
Just a basic question. I have $e^{-i9pi/4}$, and I'm struggling to understand why this is equal to $e^{-ipi/4}$.
Thanks in advance for any help.
complex-numbers
$endgroup$
Just a basic question. I have $e^{-i9pi/4}$, and I'm struggling to understand why this is equal to $e^{-ipi/4}$.
Thanks in advance for any help.
complex-numbers
complex-numbers
edited May 23 '11 at 17:38
Namaste
1
1
asked May 23 '11 at 14:07
DooodleDooodle
312
312
4
$begingroup$
Poles? $ $ $ $ $ $
$endgroup$
– Did
May 23 '11 at 14:10
add a comment |
4
$begingroup$
Poles? $ $ $ $ $ $
$endgroup$
– Did
May 23 '11 at 14:10
4
4
$begingroup$
Poles? $ $ $ $ $ $
$endgroup$
– Did
May 23 '11 at 14:10
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Poles? $ $ $ $ $ $
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– Did
May 23 '11 at 14:10
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2 Answers
2
active
oldest
votes
$begingroup$
Because
$e^{itheta}=costheta + icdot sintheta$,
$e^{-itheta}=cos(-theta)+icdot sin(-theta)=costheta - i sin(theta)$.
$sin(2pi+t)=sin(t)$, same for cosine also.
$sinleft(frac{9pi}{4}right)=sin left(2pi + frac{pi}{4}right)$.
So that
begin{align*}
e^{-ifrac{9pi}{4}}
&=cosfrac{-9pi}{4}+i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} +i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} -isinfrac{9pi}{4}\
&= cosleft(2pi + frac{pi}{4}right)-isinleft(2pi +frac{pi}{4}right)\
&=cosfrac{pi}{4}-isinfrac{pi}{4}\
&=e^{-ifrac{pi}{4}}
end{align*}
Also while doing such problems, these formulae may come handy:
$sin(pi+theta)= - sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sin(pi-theta) = sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2}+thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2} -thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi+theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi - theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2}-thetabigr)=sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2} + thetabigr)= -sintheta ; qquad 0 leq theta leq frac{pi}{2}$

$endgroup$
4
$begingroup$
So is $e^{-i5pi/4}$ also equal to $e^{-ipi/4}$, because $5 equiv 1 (text{mod} 4)$?
$endgroup$
– Jonas Meyer
May 23 '11 at 14:17
$begingroup$
@Jonas: Sorry. I made a mistake. Then i went to eat, when i realized that i had made a mistake.
$endgroup$
– user9413
May 23 '11 at 14:41
$begingroup$
So what would e−i5π/4 equal? I thought that would be e−iπ/4 as well.
$endgroup$
– Dooodle
May 23 '11 at 14:44
$begingroup$
@Doodle: For that you have to see where the Sines and cosines are positive /negative. So you have $5pi/4 = pi + pi(4)$. So $sin$ becomes negative and cos becomes positive.
$endgroup$
– user9413
May 23 '11 at 14:47
$begingroup$
@Doodle: $e^{-5pi/4}= cos(-5pi/4) +isin(-5pi/4) =cos(5pi/4)-isin(5pi/4) = cosfrac{pi}{4} +isinfrac{pi}{4}$
$endgroup$
– user9413
May 23 '11 at 14:49
|
show 4 more comments
$begingroup$
Note that $9pi/4 = (1+8)left(pi/4right) = pi/4 + 2pi$ and $e^{2pi i}=1$.
$endgroup$
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
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active
oldest
votes
$begingroup$
Because
$e^{itheta}=costheta + icdot sintheta$,
$e^{-itheta}=cos(-theta)+icdot sin(-theta)=costheta - i sin(theta)$.
$sin(2pi+t)=sin(t)$, same for cosine also.
$sinleft(frac{9pi}{4}right)=sin left(2pi + frac{pi}{4}right)$.
So that
begin{align*}
e^{-ifrac{9pi}{4}}
&=cosfrac{-9pi}{4}+i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} +i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} -isinfrac{9pi}{4}\
&= cosleft(2pi + frac{pi}{4}right)-isinleft(2pi +frac{pi}{4}right)\
&=cosfrac{pi}{4}-isinfrac{pi}{4}\
&=e^{-ifrac{pi}{4}}
end{align*}
Also while doing such problems, these formulae may come handy:
$sin(pi+theta)= - sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sin(pi-theta) = sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2}+thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2} -thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi+theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi - theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2}-thetabigr)=sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2} + thetabigr)= -sintheta ; qquad 0 leq theta leq frac{pi}{2}$

$endgroup$
4
$begingroup$
So is $e^{-i5pi/4}$ also equal to $e^{-ipi/4}$, because $5 equiv 1 (text{mod} 4)$?
$endgroup$
– Jonas Meyer
May 23 '11 at 14:17
$begingroup$
@Jonas: Sorry. I made a mistake. Then i went to eat, when i realized that i had made a mistake.
$endgroup$
– user9413
May 23 '11 at 14:41
$begingroup$
So what would e−i5π/4 equal? I thought that would be e−iπ/4 as well.
$endgroup$
– Dooodle
May 23 '11 at 14:44
$begingroup$
@Doodle: For that you have to see where the Sines and cosines are positive /negative. So you have $5pi/4 = pi + pi(4)$. So $sin$ becomes negative and cos becomes positive.
$endgroup$
– user9413
May 23 '11 at 14:47
$begingroup$
@Doodle: $e^{-5pi/4}= cos(-5pi/4) +isin(-5pi/4) =cos(5pi/4)-isin(5pi/4) = cosfrac{pi}{4} +isinfrac{pi}{4}$
$endgroup$
– user9413
May 23 '11 at 14:49
|
show 4 more comments
$begingroup$
Because
$e^{itheta}=costheta + icdot sintheta$,
$e^{-itheta}=cos(-theta)+icdot sin(-theta)=costheta - i sin(theta)$.
$sin(2pi+t)=sin(t)$, same for cosine also.
$sinleft(frac{9pi}{4}right)=sin left(2pi + frac{pi}{4}right)$.
So that
begin{align*}
e^{-ifrac{9pi}{4}}
&=cosfrac{-9pi}{4}+i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} +i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} -isinfrac{9pi}{4}\
&= cosleft(2pi + frac{pi}{4}right)-isinleft(2pi +frac{pi}{4}right)\
&=cosfrac{pi}{4}-isinfrac{pi}{4}\
&=e^{-ifrac{pi}{4}}
end{align*}
Also while doing such problems, these formulae may come handy:
$sin(pi+theta)= - sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sin(pi-theta) = sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2}+thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2} -thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi+theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi - theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2}-thetabigr)=sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2} + thetabigr)= -sintheta ; qquad 0 leq theta leq frac{pi}{2}$

$endgroup$
4
$begingroup$
So is $e^{-i5pi/4}$ also equal to $e^{-ipi/4}$, because $5 equiv 1 (text{mod} 4)$?
$endgroup$
– Jonas Meyer
May 23 '11 at 14:17
$begingroup$
@Jonas: Sorry. I made a mistake. Then i went to eat, when i realized that i had made a mistake.
$endgroup$
– user9413
May 23 '11 at 14:41
$begingroup$
So what would e−i5π/4 equal? I thought that would be e−iπ/4 as well.
$endgroup$
– Dooodle
May 23 '11 at 14:44
$begingroup$
@Doodle: For that you have to see where the Sines and cosines are positive /negative. So you have $5pi/4 = pi + pi(4)$. So $sin$ becomes negative and cos becomes positive.
$endgroup$
– user9413
May 23 '11 at 14:47
$begingroup$
@Doodle: $e^{-5pi/4}= cos(-5pi/4) +isin(-5pi/4) =cos(5pi/4)-isin(5pi/4) = cosfrac{pi}{4} +isinfrac{pi}{4}$
$endgroup$
– user9413
May 23 '11 at 14:49
|
show 4 more comments
$begingroup$
Because
$e^{itheta}=costheta + icdot sintheta$,
$e^{-itheta}=cos(-theta)+icdot sin(-theta)=costheta - i sin(theta)$.
$sin(2pi+t)=sin(t)$, same for cosine also.
$sinleft(frac{9pi}{4}right)=sin left(2pi + frac{pi}{4}right)$.
So that
begin{align*}
e^{-ifrac{9pi}{4}}
&=cosfrac{-9pi}{4}+i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} +i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} -isinfrac{9pi}{4}\
&= cosleft(2pi + frac{pi}{4}right)-isinleft(2pi +frac{pi}{4}right)\
&=cosfrac{pi}{4}-isinfrac{pi}{4}\
&=e^{-ifrac{pi}{4}}
end{align*}
Also while doing such problems, these formulae may come handy:
$sin(pi+theta)= - sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sin(pi-theta) = sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2}+thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2} -thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi+theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi - theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2}-thetabigr)=sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2} + thetabigr)= -sintheta ; qquad 0 leq theta leq frac{pi}{2}$

$endgroup$
Because
$e^{itheta}=costheta + icdot sintheta$,
$e^{-itheta}=cos(-theta)+icdot sin(-theta)=costheta - i sin(theta)$.
$sin(2pi+t)=sin(t)$, same for cosine also.
$sinleft(frac{9pi}{4}right)=sin left(2pi + frac{pi}{4}right)$.
So that
begin{align*}
e^{-ifrac{9pi}{4}}
&=cosfrac{-9pi}{4}+i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} +i sinfrac{-9pi}{4}\
&=cosfrac{9pi}{4} -isinfrac{9pi}{4}\
&= cosleft(2pi + frac{pi}{4}right)-isinleft(2pi +frac{pi}{4}right)\
&=cosfrac{pi}{4}-isinfrac{pi}{4}\
&=e^{-ifrac{pi}{4}}
end{align*}
Also while doing such problems, these formulae may come handy:
$sin(pi+theta)= - sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sin(pi-theta) = sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2}+thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$sinbigl(frac{pi}{2} -thetabigr) = costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi+theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cos(pi - theta)=-costheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2}-thetabigr)=sintheta ; qquad 0 leq theta leq frac{pi}{2}$
$cosbigl(frac{pi}{2} + thetabigr)= -sintheta ; qquad 0 leq theta leq frac{pi}{2}$

edited Dec 11 '18 at 8:04
Rócherz
3,0013821
3,0013821
answered May 23 '11 at 14:10
user9413
4
$begingroup$
So is $e^{-i5pi/4}$ also equal to $e^{-ipi/4}$, because $5 equiv 1 (text{mod} 4)$?
$endgroup$
– Jonas Meyer
May 23 '11 at 14:17
$begingroup$
@Jonas: Sorry. I made a mistake. Then i went to eat, when i realized that i had made a mistake.
$endgroup$
– user9413
May 23 '11 at 14:41
$begingroup$
So what would e−i5π/4 equal? I thought that would be e−iπ/4 as well.
$endgroup$
– Dooodle
May 23 '11 at 14:44
$begingroup$
@Doodle: For that you have to see where the Sines and cosines are positive /negative. So you have $5pi/4 = pi + pi(4)$. So $sin$ becomes negative and cos becomes positive.
$endgroup$
– user9413
May 23 '11 at 14:47
$begingroup$
@Doodle: $e^{-5pi/4}= cos(-5pi/4) +isin(-5pi/4) =cos(5pi/4)-isin(5pi/4) = cosfrac{pi}{4} +isinfrac{pi}{4}$
$endgroup$
– user9413
May 23 '11 at 14:49
|
show 4 more comments
4
$begingroup$
So is $e^{-i5pi/4}$ also equal to $e^{-ipi/4}$, because $5 equiv 1 (text{mod} 4)$?
$endgroup$
– Jonas Meyer
May 23 '11 at 14:17
$begingroup$
@Jonas: Sorry. I made a mistake. Then i went to eat, when i realized that i had made a mistake.
$endgroup$
– user9413
May 23 '11 at 14:41
$begingroup$
So what would e−i5π/4 equal? I thought that would be e−iπ/4 as well.
$endgroup$
– Dooodle
May 23 '11 at 14:44
$begingroup$
@Doodle: For that you have to see where the Sines and cosines are positive /negative. So you have $5pi/4 = pi + pi(4)$. So $sin$ becomes negative and cos becomes positive.
$endgroup$
– user9413
May 23 '11 at 14:47
$begingroup$
@Doodle: $e^{-5pi/4}= cos(-5pi/4) +isin(-5pi/4) =cos(5pi/4)-isin(5pi/4) = cosfrac{pi}{4} +isinfrac{pi}{4}$
$endgroup$
– user9413
May 23 '11 at 14:49
4
4
$begingroup$
So is $e^{-i5pi/4}$ also equal to $e^{-ipi/4}$, because $5 equiv 1 (text{mod} 4)$?
$endgroup$
– Jonas Meyer
May 23 '11 at 14:17
$begingroup$
So is $e^{-i5pi/4}$ also equal to $e^{-ipi/4}$, because $5 equiv 1 (text{mod} 4)$?
$endgroup$
– Jonas Meyer
May 23 '11 at 14:17
$begingroup$
@Jonas: Sorry. I made a mistake. Then i went to eat, when i realized that i had made a mistake.
$endgroup$
– user9413
May 23 '11 at 14:41
$begingroup$
@Jonas: Sorry. I made a mistake. Then i went to eat, when i realized that i had made a mistake.
$endgroup$
– user9413
May 23 '11 at 14:41
$begingroup$
So what would e−i5π/4 equal? I thought that would be e−iπ/4 as well.
$endgroup$
– Dooodle
May 23 '11 at 14:44
$begingroup$
So what would e−i5π/4 equal? I thought that would be e−iπ/4 as well.
$endgroup$
– Dooodle
May 23 '11 at 14:44
$begingroup$
@Doodle: For that you have to see where the Sines and cosines are positive /negative. So you have $5pi/4 = pi + pi(4)$. So $sin$ becomes negative and cos becomes positive.
$endgroup$
– user9413
May 23 '11 at 14:47
$begingroup$
@Doodle: For that you have to see where the Sines and cosines are positive /negative. So you have $5pi/4 = pi + pi(4)$. So $sin$ becomes negative and cos becomes positive.
$endgroup$
– user9413
May 23 '11 at 14:47
$begingroup$
@Doodle: $e^{-5pi/4}= cos(-5pi/4) +isin(-5pi/4) =cos(5pi/4)-isin(5pi/4) = cosfrac{pi}{4} +isinfrac{pi}{4}$
$endgroup$
– user9413
May 23 '11 at 14:49
$begingroup$
@Doodle: $e^{-5pi/4}= cos(-5pi/4) +isin(-5pi/4) =cos(5pi/4)-isin(5pi/4) = cosfrac{pi}{4} +isinfrac{pi}{4}$
$endgroup$
– user9413
May 23 '11 at 14:49
|
show 4 more comments
$begingroup$
Note that $9pi/4 = (1+8)left(pi/4right) = pi/4 + 2pi$ and $e^{2pi i}=1$.
$endgroup$
add a comment |
$begingroup$
Note that $9pi/4 = (1+8)left(pi/4right) = pi/4 + 2pi$ and $e^{2pi i}=1$.
$endgroup$
add a comment |
$begingroup$
Note that $9pi/4 = (1+8)left(pi/4right) = pi/4 + 2pi$ and $e^{2pi i}=1$.
$endgroup$
Note that $9pi/4 = (1+8)left(pi/4right) = pi/4 + 2pi$ and $e^{2pi i}=1$.
edited May 23 '11 at 14:23
Eric Naslund
60.7k10140242
60.7k10140242
answered May 23 '11 at 14:14
lhflhf
167k11172403
167k11172403
add a comment |
add a comment |
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Poles? $ $ $ $ $ $
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– Did
May 23 '11 at 14:10