I don't understand what's written here:

[![enter image description here](https://i.stack.imgur.com/Tt78u.jpg)](https://i.stack.imgur.com/Tt78u.jpg)

The point I don't understand is: $\cos (s+t)$. How can we consider the cosine of $s+t$ when triangle $\triangle P\_1OP\_0$ is not even a right triangle? As far as I understand, a cosine (as well as sine, tangent, etc.) is a property of a right triangle; that is, a triangle, in which one corner is equal to $90$ degrees. But none of the angles in $\triangle P\_1OP\_0$ is a right angle.