Some statements which hold for all angles are shown in Figure T-15.
- From complex analysis we learn that a complex number
- is called the modulus
- is the called the phase
Complex exponential form of sine and cosine
Double angle formulas
Cosine and Sine of a sum and difference of angles
Products of Cosines and Sines
Deriving the identities
We can show that it is entirely plausible that the identities above are true.
We begin by formally writing the Taylor series representations of
and and sum the resulting series
where We may write the Taylor series form of
where . In both of the previous cases, free use has been made of the identity
The sum of these two series yields the series representation of the exponential function
This is not a proof of Euler's relation. It is only an argument of plausibility, because we have not proven the many theorems
that would allow us to establish the existence of the complex exponential, the Taylor series, or its convergence. Rarely will \thetas
argument of plausibility appear in a textbook on complex analysis, owing to its heuristic nature.
Complex exponential form of trigonometric functions
and that, owing to the eveness of cosine and the oddness of sine,
we may write the complex conjugate of as
The complex exponential form of sine and cosine follow from the respective sum and difference of
The other identities derive from the complex exponential form of sine and cosine.