recursive formula for points on the unit circle of regular 2^n gon's

I found that formulas of this type:

describe the points on a circle corresponding to regular 2^(n+1)-gons where n is the number of square root symbols in the formulas and a,b,c... are each 1 or 2. For example a 16-gon

I haven't quite figured out where each point will be on the unit circle by the signs of the terms though.