What is Leonard Euler's famous formula?

Answer 1

eix = cos(x) + i*sin(x)

where e is the irrational number 2.7182...

i is the maginary sq root of -1

and x is measured in radians.

In the special case when x = pi, this reduces to:

eiπ = cos(π) + i*sin(π)

or

eiπ = -1