[kwertzzz]

Here is a "Proof of the Law of Cosines" by the same author:

https://www.tandfonline.com/doi/pdf/10.4169/amer.math.monthl...

But this time not with complex numbers.

[mehulashah]

So, I have to pay to read the proof? Not fun.

[milesde]

It's also on JSTOR: https://www.jstor.org/stable/10.4169/amer.math.monthly.121.0... . No payment needed there (unless you've burned through your 100 free articles for this month).