I Annoying detail in derivation of Compton scattering

[crossword.bob]

TL;DR Summary

Looking for an explanation of one term in the Compton's original derivation of X-ray scattering

In Compton's 1923 paper on X-rays scattering from light elements, he presents the following diagram:

Here, $h\nu_0/c$ is the momentum of the incident photon, $h\nu_\theta/c$ is that of the scattered photon and $mv/(1-\beta^2)^{1/2}$ is that of the recoiled electron. He uses this to express conservation of momentum as $$\left(\frac{m\beta c}{\sqrt{1-\beta^2}}\right)^2 = \left(\frac{h\nu_0}{c}\right)^2 + \left(\frac{h\nu_\theta}{c}\right)^2 + 2 \frac{h\nu_0}{c}\cdot\frac{h\nu_\theta}{c}\cos\theta.$$

I probably shouldn't obsess over details here, but the plus sign on the $\cos$ term is annoying me. Is this not just a simple invocation of the cosine rule? Is one of the terms taken to be negative?

 

[crossword.bob]

Sorry, I’ve now seen that this is a known thing. I’ll work on my search-fu.