de Moivre identity, proof of
To prove the de Moivre identity, we will first prove by induction
on n
that the identity holds for all natural numbers
.
For the case n=0, observe that
cos(0θ)+isin(0θ)=1+i0=(cos(θ)+isin(θ))0. |
Assume that the identity holds for a certain value of n:
cos(nθ)+isin(nθ)=(cos(θ)+isin(θ))n. |
Multiply both sides of this identity by cos(θ)+isin(θ) and expand the left side to obtain
cos(θ)cos(nθ)-sin(θ)sin(nθ)+icos(θ)sin(nθ)+isin(θ)cos(nθ) | =(cos(θ)+isin(θ))(cos(nθ)+isin(nθ)) | ||
=(cos(θ)+isin(θ))n+1. |
By the angle sum identities,
cos(θ)cos(nθ)-sin(θ)sin(nθ) | =cos(nθ+θ) | ||
cos(θ)sin(nθ)+sin(θ)cos(nθ) | =sin(nθ+θ) |
Therefore,
cos((n+1)θ)+isin((n+1)θ)=(cos(θ)+isin(θ))n+1. |
Hence by induction de Moivre’s identity holds for all natural n.
Now let -n be any negative integer. Then using the fact that cos is an even and sin an odd function, we obtain that
cos(-nθ)+isin(-nθ) | =cos(nθ)-isin(nθ) | ||
=cos(nθ)-isin(nθ)cos2(nθ)+sin2(nθ) | |||
=1cos(nθ)+isin(nθ)⋅cos(nθ)-isin(nθ)cos(nθ)-isin(nθ) | |||
=1cos(nθ)+isin(nθ), |
the denominator of which is (cos(nθ)+isin(nθ))n. Hence
cos(-nθ)+isin(-nθ)=(cos(θ)+isin(θ))-n. |
Title | de Moivre identity, proof of |
---|---|
Canonical name | DeMoivreIdentityProofOf |
Date of creation | 2013-03-22 14:34:08 |
Last modified on | 2013-03-22 14:34:08 |
Owner | rspuzio (6075) |
Last modified by | rspuzio (6075) |
Numerical id | 10 |
Author | rspuzio (6075) |
Entry type | Proof |
Classification | msc 12E10 |
Synonym | proof of de Moivre’s formula![]() |
Synonym | proof of de Moivre’s theorem |
Related topic | AngleSumIdentity |