1. The problem statement, all variables and given/known data Currently revising for a differential geometry exam. The question I am working on is one of those types where the next part of the question follows from the last. I've gotten to the point where I have proven T⋅dNds=−κ, 2. Relevant equations The next part is where I got stuck, which is to prove dN/ds=−κT+τB. I looked at the mark scheme 3. The attempt at a solution it said "Follows from previous item, and B=T×N". I simply don't see how it follows, though.