There is no equation to look at. You just have to look at the angles. to find the first angle they are using (m∠DXB) Put your finger on D then slide it to the next letter X and then move it to the letter B. Hopefully you found the angle by now..
This angle that you just found is approximately 2/3 of the entire angle.
Now can you say 2/3 + 1/3 = 3/3 or 1 whole.
so our approximate 1/3 of the entire angle is m∠BXA
So start by looking at B and then moved your finger to X then moving it to A. And you should find the open triangle shape.
Now When you add those together you get one whole. So this entire open triangle is what we need to write down.
