1. Derivation of Euler's formula 
2. Geometric derivation of Euler's formula
2.1 When two complex numbers are multiplied, the auxiliary angle of the new complex number is equal to the addition of the auxiliary angles of the two complex numbers, and the module is equal to the multiplication of the two modules.
2.2 Finally, a semicircle with radius 1
3. Application of Euler's formula
When Euler's formula is not pi, but any angle and the angle changes with time, it can be used for rotation angular velocity.