Attached image with problem. 2 Questions; 1. δn.|AB| = δx.|BC|.cosΘ becomes δn = δx.|BC|.cosΘ / |AB| This is dividing the entire "δx.|BC|.cosΘ" equation by |AB| or just the |BC| part? Is there a difference? 2. Half way down page, boxed equation. How does δx.|BC|.cosΘ / |AB| = δxcos^2Θ and then 1/2(δx+δy) Is this calculus? Should I find this in a log table I've been searching but haven't found where to look this up. Thanks for any help.