50g

First of all, I am sorry the symbols I put before didn't show up in the 
replay I posted, so here it goes again ("->" means the arrow and "d" is 
the differential:
what is happening with your equations is that the F(x) in N(x)
is different from the F(x) you define, for example when you define f
(x)=x^3-7 the calculator creates a program that looks like this:
<< ->¨ x 'x^3-7' >>
and stores it in a global variable called 'F'; this means that in run-
time the calculator create a temporary local variable called 'x' this
variable is different that a global variable called 'x' that you may
have in your current directory. The same is true for the F(x) you
define and F(x) in N(x). A better approach is to have a program N like
this:
<< -> x0
<< 'x' DUP F DUP 'x' d / - 'x' x0 = SUBST ->NUM >>
>>

For the second question I have no clue,  nothing seems to work, I am 
posting your question in hp forums, I hope they have an answer.