I am reading an analysis on circuit board trace heating, and have found a source for a general temp rise vs current equation that matches measured IPC 2152 closely. However, I am having trouble following the steps from Eq. 2-3 to Eq. 3-1. I understand that the trace resistance is inversely proportional to the area, but don't know how the model picks up the a1, a2 exponents. Is this just a curve fitting thing or is it some algebraic trickery?