You might be wondering why at 7:04 i integrated only at one side of the equation, If you look closely , i integrated on both the sides. SAY HI TO ME ON MY NEW INSTAGRAM ! https://www.instagram.com/sayanseal3 By changing the notations on the left hand side that is by removing the( fi and theta) angle from the suffix of (dn) i integrated the left hand side also with respect to d(fi) and d(theta). See the different notations on the Right hand side of equation at 6:52 and the equation above it; pray to god and stay healthy everyone. Music credits:Bensound.com(Check them out) Thanks for watching;