Euler's Formula for Complex Numbers
(There is another "Euler's Formula" about Geometry,
this page is about the one used in Complex Numbers)
First, you may have seen this famous equation:
e^{iπ} + 1 = 0
It seems absolutely magical that such a neat equation combines:
 e (Euler's Number)
 i (the unit imaginary number)
 π (the famous number pi that turns up in many interesting areas)
 0 and 1 (also wonderful numbers!)
But if you want to take an interesting trip through mathematics, then read on to find out why it is true.
Euler's Formula
It actually comes from Euler's Formula:
e^{ix} = cos x + i sin x
When we calculate that for x = π we get:
So e^{iπ} + 1 = 0 is just a special case of a much more useful formula that Euler discovered.
Discovery
It was around 1740, and mathematicians were interested in imaginary numbers.
An imaginary number, when squared gives a negative result
This is normally impossible (try squaring any number, remembering that multiplying negatives gives a positive), but just imagine that you can do it, call it i for imaginary, and see where it carries you:
i^{2} = 1
Euler was enjoying himself one day, playing with imaginary numbers (or so I imagine!), and he took this Taylor Series (which was already known):
(You can use the Sigma Calculator to play with this.)
And he put i in it:
And because i^{2} = 1, it simplifies to:
Now, separating the terms without and with i gets:
And here is the miracle ...

So we get:
Example: when x = 3
Note: we are using radians, not degrees.
The answer is a combination of a Real and an Imaginary Number, which together is called a Complex Number.
We can even plot such a number on the complex plane (the real numbers go leftright, and the imaginary numbers go updown):
Here we show the number −0.990 + 0.141 i
Which is the same as e^{3i}
A Circle!
In fact, putting Euler's Formula on that graph produces a circle:
e^{ix} produces a circle of radius 1
And we can turn any point (such as 3 + 4i) into re^{ix} form (by finding the correct value of x and the radius, r, of the circle)
Example: the number 3 + 4i
To turn into re^{ix} form we do a Cartesian to Polar conversion:
 r = √(3^{2} + 4^{2}) = √(9+16) = √25 = 5
 x = tan^{1} ( 4 / 3 ) = 0.927 (to 3 decimals)
So 3 + 4i can also be 5e^{0.927 i}
There are many cases (such as multiplication) where it is easier to use re^{ix} than a+bi
Lastly, here is the point created by e^{iπ} (where our discussion began):
e^{iπ} = −1