1. The problem statement, all variables and given/known data i have to show that the lenght of line integral squared on surface (ds^2) , which has vectorequation r=r(u,v) (r here is a vector), can be given as ds^2=Edu^2+2Fdudv+Gdv^2 (Find functions E,G,F) 2. Relevant equations I should use diferential dr=du(t)e1+dv(t)e2 (dr, e1 and e2 here are vectors ) ds=|dr/dt|dr (dr here is vector) dS=dS/n (dS/n is a vector ) 3. The attempt at a solution I'm pretty much stuck at the beginning , i don't see how those relevant equations help me here ? Probably i should do something with dS/n. Sorry for bad english and possibly translation, point out mistakes so i can learn.