Unaware how to do that type of math myself, but just trying to be helpful...
The explanation at your posted link appears to have been written for a different field than electronics. Whether or not that's the reason for the discrepancy, it is common in our schooling to be exposed to explanations that:
* are based on greater requisite knowledge than we have right now
* demonstrate a method primarily rather than an equation, in the intention that we will adapt the method to our own use
* were lifted from a treatise which contains a few missing details in another chapter, and that chapter was omitted from the article you're reading.
* are really too complicated to present in a linear manner, yet we can only read in a linear manner
* are about a topic that needs to be seen from different angles, to get a feel for the ways in which one branch joins to other branches, so to speak.
* are not entirely correct now, but may have been at an earlier time