we do that just to confuse you! and making the proper greek symbols is beyond my patience level
T = torque on shaft
r = radius of shaft
d = diameter of shaft
J = polar moment of inertia of shaft
And there are some assumptions made when the forumula is derived, so having a mechanics of materials book or a mechanical engineering handbook that explains it would be nice...but that's really 3rd year engineering school stuff....nice to see you're getting a head start