, and then add and subtract these two formulas to get:

A formula connecting complex exponential function and trigonometric function,

E is the base of natural logarithm, and I is the imaginary unit. It extends the domain of exponential function to complex numbers,

Euler Formula (3 sheets)

The relationship between trigonometric function and exponential function is established, which not only appears in mathematical analysis, but also occupies a very important position in the theory of complex variable function, and is also called "overpass in mathematics". ?