Some things really test my faith in math
and physics.  Consider the following
example, use notation that ax/at means
partial of x wrt t. OK.  What is the
difference between this: a/ar[ a(phi)/ar ]
and this: 1/r^2 a/ar[ r^2 a(phi)/ar ] ?  If
you say nothing, then you are wrong.  
Annoyed?  So am I.  The only difference is
one will give the right answer to a problem
and the other is garbage.