HP Solve Equation Library application card

Hewlett-Packard Journal, June, 1991 by Eric L. Vogel

Messages appear during the solution identifying which variable is being solved for and its resulting value. After the solution has completed, each variable retains its initial state. Correspondingly, each menu key retains its initial appearance. Black keys (knowns) remain black, and white keys (unknowns) remain white. This simplifies solving a problem using the same knowns and unknowns but with different values.

Indicating Variable Relationships

After a solution, some of the menu keys will have a small block in them to indicate the roles their variables played in the solution (Fig. 4c). A block in a black key (known) indicates that the variable was used to find an unknown in a particular equation. A block in a white key (unknown) indicates a value was calculated for that variable during the solution. This represents a unique state for a variable-it is an unknown, yet it has a calculated value. The next time this variable is solved for, this calculated value will be used as its initial guess.

Pressing the shift key followed by a menu key solves for that specific variable, regardless of whether it is black or white (known or unknown). After a variable is solved for, its menu key is shown in white with a block to indicate an unknown that was solved for with a calculated value (Fig. 4d). Other menu keys may have blocks in them based on the roles their variables played in the solution. Solution Summary

A summary of the solution procedure is available by pressing the shift key followed by the ALL key (Fig. 5). This summary shows a catalog of each unknown that was found, its calculated value, and the equation that was solved to find it, in the order that the unknowns were determined by the solution procedure.

Acknowledgments

I would like to thank the rest of the development team and the people who made exceptional contributions to the product: Jim Donnelly and Megha Shyam for making it all happen, Dennis York for the initial vision, Lynn Winter for a catalog engine that was an enduring foundation, Chris Bunsen for critical design advice and testing for the Multiple Equation Solver, Diana Byrne for spearheading the software testing effort, Hank Schroeder for an owner's manual in a remarkably short time, and Ray Depew, whose thoroughness testing the Periodic Table was critical to both the card and the HP 48SX.