Hi guys! I hope someone here will be able to help me with this question:

f(r) is a scalar field, then use stoke's theorem (∫∫c curlF.dS=∫cF.dr) to deduce that:

∫∫s grad(f) x dS= -∫c f dr

I am stuck as to how to do this. I have tried subbing in the vector calculus identity curl(fu)=fcurl(u)+(gradf)xu (where u is a constant). Thus

∫∫s grad(f) x dS =∫∫scurl(fu) - fcurl(u) (where dSis u)

but I am not sure whether this is right (and i then get stuck at this point too). Please help!