log(a*b) = log(a) + log(b)

Or: how to transform a multiplication into an addition.

  i^2 = -1

Or: multiplying by i is a 90° rotation on the plane

This was an eye opener for me. Math is designed to simplify things. To represent concepts in an easy way to read and manipulate and tinker with.

I tried at some point to invent an extension of the complex numbers to represent a 3D space with i and j, only to rediscover that this is not possible, you need to go deeper, with i, j and w to represent 3D rotations along an axis. and that's how i began reading about quaternions.

The beauty of maths is also the fact that it is kind of universal, even if it was invented.