Probability theory, is a mathematical branch that has to do with how random phenomena are analysed. To solve for probability, one has to divide the number of events aggregate by the amount of possible outcomes.
We should note that the statement in the question regarding probability theory providing mathematical rules that are assigned probabilities to the outcomes of the random experiments is said to be true.
The sides all satisfy the Pythagorean Theorem so it is a right triangle. I attached an image of how the triangle should look like and as you can see, the angle is 90 degrees or pi/2 radians.
