# Euler's formula

Euler's formula is a relationship between exponents of imaginary numbers and the trigonometric functions: For example, if , then ### Relationship to sin and cos

In Euler's formula, if we replace θ with -θ in Euler's formula we get  and we get or equivalently, Similarly, subtracting from and dividing by 2i gives us: Multiplying the top and bottom by -i gives us: These formulas allow us to define sin and cos for complex inputs. The existence of these formulas allows us to solve 2nd order differential equations like

y" + y' + y = 0

using sines and cosines.

Example

Find sin(3 + 4i) using Euler's formula:

Using the formula derived above, we plug 3 + 4i in for θ: = = From Euler's formula,  Plugging these into the formula for sin(3 + 4i) yields:     ### Proof of Euler's formula

Given the Maclaurin series for ex, cos(x), and sin(x):

 ex = = sin(x) = = cos(x) = = Notice that if we plug ix into the Mauclaurin series of ex we get    After repeated multiplication, i cycles through i, -1, -1, 1, and back again to i, so i has a period of 4. As a result, the terms in the above series switch signs after every 2 terms. If we group the even and odd powers together, we get   Notice that the terms in parentheses above are equivalent to the Maclaurin series for cos(x) and sin(x) respectively, so plugging cos(x) and sin(x) for their respective Maclaurin series in the above equation yields:

eix = cos(x) + i sin(x)