hp 35s  Entering and Evaluating Equations
The HP35S can be used for specifying an equation to evaluate. Suppose there is a frequent need to determine the volume of a straight pipe. The equation is
V = .25 pi d2 l
Where d is the inside diameter of the pipe, and l is its length. The calculation could be keyed in over and over.
For example:
.25 {ENTER} {RightShift} {pi} {x} 2.5 {x2} {x} 16 {x} calculates the volume of 16 inches of a 2 1/2inch diameter pipe (78.5398 cubic inches). However, by storing the equation, the calculator can ‘remember’ the relationship between diameter, length, and volume  so it can be used many times.
Put the calculator in equation mode and type in the equation using the following keystrokes:

Press {RightShift}, then {EQN} Display [ EQN LIST TOP ] or the current equation. Selects Equation mode, shown by the EQN annunciator.

Press {RCL} Dispaly [ _ ] Begins a new equation, turning on the equationentry cursor. RCL turns in the A…Z annunciator so a variable name can be entered.

Press {V}, {RightShift}, then {=} Display [ V = ‘_’ ] RCL, V types V and moves the cursor to the right.

Key in.25 Display [ V = 0.25_ ] Digit entry uses the ‘_’ digitentry cursor

Press {x}, {RightShift}, {pi}, then {x} Display [ V = 0.25 x pi x ‘_’ ] x ends the number and restores the ‘_’

Press {RCL}, {D}, {yx} , then {2} Display [ = 0.25 x pi x D ^ 2 ‘_’ ] yx types ^.

Press {x}, {RCL}, then {L} Display [ 0.25 x pi x D^2 x L ‘_’ ] where V = scrol{LeftShift} off the left side of the display

Press {ENTER } it will display [ v = 0.25xpixD^2x ] Terminates and displays the equation. The right arrow shows that part of the equation does not fit in the display, and the symbol above SUM+ means press SUM+ to see characters in that direction.

Hold {RightShift} and {SHOW} display [ CK = 5836 026.0 ] Shows the checksum and length for that equation, so keystrokes can be checked.
By comparing the checksum and length of the equation with those in the example, it can be verified that the equation has been entered properly.
Evaluate the equation (to calculate V):

Press{ ENTER} it will display [ D? value ] Prompts for variables on the righthand side of the equation. Prompts for D first; value is the current value of D

Key in 2.1.2 Display [ D? 2 1/2_ ] Enters 2 1/2 inches as a fraction

Press {RightShift} Display [ L? value ] Stores D, prompts for L; value is current value of L

Press 16, then {RightShift} Display [ v = 78.5398 ] Stores L; calculates V in cubic inches and stores the result in V
These procedures are also applicable for HP 33s.