I remember that it should be in the high school Olympic math book, but there is no detailed explanation. This is taught in universities, but it belongs to elementary mathematics, not to advanced mathematics. The triangular form of complex number can be understood by the geometric meaning of vector, and the exponential form of complex number can be analogized by the multiplication law of triangular form and exponential multiplication law. However, the derivation of e requires partial differentiation on both sides (belonging to high numbers and involving Euler formula). In a word, this knowledge can be regarded as the foundation for learning complex variable functions.