Derivatives on HP 42S

[Thomas Klemm]

Not sure if I understand you correctly but you can split the program into:

Function

Code:

LBL "Fx"
SIN
LASTX
÷
END

Derivative

Code:

LBL "dF"
MVAR "x"
MVAR "h"
RCL "x"
RCL "h"
COMPLEX
XEQ "Fx"
COMPLEX
END

Set MODES to RAD.
Start SOLVER.
Select Solve Program: dF
Set small value h: 1E-6 h
Set lower guess: π x
Set upper guess: π 2 × x
Solve for x: x

x=4.493409

Exit the solver with:
EXIT
EXIT

Now you can run:
XEQ "Fx"

x: -0.217234

Is this what you wanted to achieve?

Kind regards
Thomas