A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robert Bernecky
Date: 2017 Jul 18, 06:49 -0700
That last step is an approximation. If you know calculus and Taylor Series (see wikipedia for more info), you can expand
1/(1-x)^2 = 1+ 2x + 3x^2 + 4x^3 + ...
about the value x=0. In this case, x is epsilon*cos().
For this problem, Geoge dropped all the "high order" terms, to get the approximation 1+2x