HP 50g Graphing manual InverseFunction Solver, Techniques used in Rootr, Rootr program listing

InverseFunction Solver

This section describes the program ROOTR, which finds the value of x at which f(x) = y. You supply the variable name for the program that calculates f(x), the value of y, and a guess for x (in case there are multiple solutions).

Level 3	Level 2	Level 1	→	Level 1
'function name'	y	xguess	→	x

Techniques used in ROOTR

Programmatic use of rootfinder. ROOTR executes ROOT to find the desired xvalue.

Programs as arguments. Although programs are commonly named and then executed by calling their names, programs can also be put on the stack and used as arguments to other programs.

ROOTR program listing
		Program:	Comments:
		«
		→ fname yvalue xguess	Creates local variables.
		«	Begins the defining procedure.
		xguess 'XTEMP' STO	Creates variable XTEMP (to be solved for).
		« XTEMP fname	Enters program that evaluates f(x) y .
		yvalue »
		'XTEMP'	Enters name of unknown variable.
		xguess	Enters guess for XTEMP.
		ROOT	Solves program for XTEMP.
		»	Ends the defining procedure.
		'XTEMP' PURGE	Purges the temporary variable.
		»
		`OROOTR K	Stores the program in ROOTR.
Checksum: # 4708d
Bytes:	163

Example: Assume you often work with the expression

3.7x3 + 4.5x2 + 3.9x + 5 and have created the program X→FX to calculate the value:

«→ x '3.7*x^3+4.5*x^2+3.9*x+5' »

You can use ROOTR to calculate the inverse function.

238 RPL Programming Examples