HP -28S Quick Reference
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HP-28S
The solver finds a solution for x (or y) where the left side equals the
right side.
• A program, ie. << X DUP * 7 – Y + >>.
The solver finds the value for one of the variables used in the
program (X or Y) for which the program returns 0 in stack level 1.
The program must not take anything from the stack.
Note that the solver and plot commands (see PLOT Menu) use the same
equation EQ.
RCEQ Recall the function stored in variable EQ.
SOLVR:
Variables
Display the interactive solver menu with all the independent variables
used in the function. The menu looks different than normal command
menus in that the names are printed black on a white background.
While the interactive menu is active the stack can be used normally.
• An independent variable is either a formal variable (with no
associated data) or a variable containing data (ie. a number).
Variables containing procedures do not show up in this menu. Instead
the independent variables of these procedures are listed. If any one
of these nested procedures is modified the solver menu is updated
automatically.
Note that if a variable contains an equation the "=" sign is replaced
by "-" (minus) in order to convert it to an expression which returns a
result.
By pressing the menu button associated with a variable a value can be
stored for the variable:
• The value is taken from stack level 1 as usual.
• The stored value is also temporarily displayed in the top line of the
LCD display.
continued
SOLVR:
Initial Guess
For the variable that is solved for an initial guess must be specified:
• This can be a single number or list with a single value.
• Or a list with two values which indicate the initial interval of the
search algorithm. If the function has a different sign at these two
points the solver will quickly find a root.
• Or a list with three values where the first is a best guess and the
other two values should be below and above the best guess.
• Note: After using DRAW on the function in EQ and looking at the
function curve it is usually simple to see initial guesses for the solver.
DRAW offers a cross-hair pointer that can be moved over the plot and
complex (x,y) coordinates can be selected and stored on the stack.
These complex values can be used as initial guesses (the imaginary
part will be ignored). See PLOT Menu.
continued
SOLVR:
Finding the
root
Pressing SHIFT and one of the menu buttons invokes the solver for the
specified variable.
• The solver can be interrupted by pressing ON. In this case a best
guess in the form of a length-3 list is returned.
• Pressing any other key displays the current best guess in lines 2 and
3 of the LCD display and the root finding process continues.
continued
SOLVR:
Results
When done the solver will return the result in stack level 1 and also
display it in the display line 1. In display line 2 one of these messages
appears:
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