Hi again,
From that paper without me going into detail i would think that like any other paper you should be able to trace the equations backwards and find out the origin of the expression as to how they arrived at that one. For your link example, starting with equation 1.2.34 you can see just above that they state, "Now from equation 1.2.32 we can obtain total modulation index m1", so you would go back and find equation 1.2.32 and figure out how they got 1.2.34 from that, then go back before 1.2.32 and find out how they got that, and keep going back and trying to understand how they got from one equation to the next and eventually you should get back to some more generally understood equation or concept. That process should show you how they got to that simpler equation.