I was going the derivations for Fresnel zone radius approximation, and there was a jump in the math which I don't fully understand. If someone could take a look at this and help me figure. I was hinted that it had to do with the binomial theorem, but I have no idea >.< Seems like LaTeX isn't working -_- (D^2 + r^2)^(1/2) if r << D ( D^2( 1 + (r^2)/(D^2) ) )^(1/2) D1 + (r^2)/(2D^2) <--- ??? . . .