Euler's Formula
Euler’s formula returns a complex number on the unit circle corresponding to the input angle (theta).
Expression  Description 

The exponential function which is sometimes written in the shorthand form .  
The complex constant . See complex numbers.  
The angle provided in radians.  
The cosine function.  
The sine function. 
Euler’s formula takes an angle as input and returns a complex number that represents a point on the unit circle in the complex plane that corresponds to the angle. For example, given the angle of radians, Euler’s formula returns the complex number which is the rightmost point on the unit circle in the complex plane.
Note, the notation is shorthand for the exponential function. Shown below is the formula written explicitly with the exponential function.
When using a computational medium that supports complex numbers this is useful to know. Conceptually, the definition of the exponential function can be used to verify the formula as discussed in the explanation below.
The connection between the exponential function and the trigonometric functions sine and cosine is surprising and gives this formula notoriety. However, as mentioned above is shorthand for the exponential function.
The example below derives Euler’s formula starting with the power series definition of the exponential function^{[1]}.
Start with the power series definition of the exponential function.
Substitute the complex input into the function as input.
Expand the expressions in the numerators.
Notice the places where the complex constant appears. Everywhere the complex constant is raised to a power greater than one, such as , and we can substitute into the expression.
Simplify the expressions which flip some of the signs. The expressions that still contain the constant are highlighted in blue.
Group the expressions containing and those that do not and then factor out the complex constant.
Substitute the definitions into the expression.
This gives us Euler’s formula which we can write in its shorthand form shown below.

Derive Definition of Exponential Function (Taylor Series) Example

Derive Sine Function (Taylor Series) Example

Derive Cosine Function (Taylor Series) Example