HP 95LX - Page 57

Combining Two or More Equations
Sometimes sets of shorter equations share some variables, and it would be
convenient to have access to them all on a one single menu. You can do this, by
using a special function—the “If Solving” function. For example, you can combine
these three short related equations into one calculation group:
D=— n=— PV =nRT
Vv Zz
Use the If-Solving function to do it:
IFCSCD) yD—-Cm~ YD, (“IF Solving for D, solve D—{m-Y¥>=0,
IF(S(m) yCm~Z>-Ny (“or,if Solving for M, solve {M+ ZI —N=8,
) P¥V-n*xR*T (“otherwise, solve P¥Y—n*R*T=8.")
=a
Notice three important things about the structure:
® The commas between the component sections are mandatory.
e The ending parentheses (one to close each IF (5... > function) are required.
* You must rearrange each short equation so that it’s set to equal zero, and then
you put a =@ added to the end of the expression.
When you press (MERE) to use this combined equation, all of the variables
appear on the same display (in the order they first appeared in the equation).
* You can solve for D only after both m and V are known;
* You can solve for m only after both n and Z are known;
* You can solve for P, V, n, R, or T only when four of them are known;
* You cannot solve for Z at all—it is strictly an input, given the logic of the If
Solving functions used. You can fix this by addinga IF{SC{ZJ 4... function
into the solving progression, if you need to.
Be careful when using combined equations that you are able to solve all the
variables you need. And—as always—be sure to test a newly-written equation
with known data and solutions before trusting it.
Using the Solver 57