[BUG] common student errors

[Tevian Dray]

Here are a couple of commone student misconceptions from my multivariable
calculus class last term:

* dA vs d(A)

Some students want dA to be d of something.  So on a sphere, for instance,
since A=4*Pi*r^2, dA must be 8*Pi*r*dr.  I guess we need to better emphasize
that "dA" (and "dV") are not differentials.

* gradients and optimization

I like to emphasize that grad(f)=0 (or is undefined) at a critical point; this
formulation is coordinate-independent.  I also point out that, in the case of
one constraint, the method of Lagrange multipliers is equivalent to
determining where the cross product of the gradients is zero.  However, these
very nice geometric properties confuse some students, since they then mix
these two conditions up.

Tevian