     TOPIC: Prompted variables in SOLVER 
     PRODUCT:  TI-85 
     DATE: September 1, 1992 
 
     This note is posted in response to a question posted by Benjamin J. 
     Sultenfuss regarding the illumination equation example using the 
     equation solver (pages 9 - 12, Getting Started in TI-85 manual). 
 
     The basic question was why the variables HEIGHT and BASE were not 
     shown on the SOLVER edit screen when Mr. Sultenfuss changed the 
     example slightly from the way it is shown in the manual.  After 
     step 2 shown at the bottom of page 9 of the manual, he entered the 
     expressions:  BASE=6:HEIGHT=8 on the home screen.	This action 
     caused BASE and HEIGHT to not be listed as variables in the later 
     equation for ILLUM as the example continues on page 10. 
 
     On the TI-85, the expression: 6 -> BASE   (the -> is the STO key) 
     will store the numeric value 6 to variable BASE.  However, the 
     expression:  BASE=6  is an equation in its own right.  So when the 
     calculator processes an equation:	ILLUM=INTEN*HEIGHT/DIST^3 , 
     where DIST=sqrt(BASE^2 + HEIGHT^2) , all the variables, INTEN, HEIGHT, 
     BASE which represent real numbers are shown to allow entry or 
     calculation.  ILLUM is also shown because it is the value for the 
     equation.	DIST is not shown, because it is an equation.  If, for 
     example, we stored 6 -> DIST, it would no longer be an equation, but 
     a real number variable.  For this reason, we cannot solve for a 
     numeric value for DIST since it would replace the equation.  We 
     can solve for BASE and HEIGHT, which will determine DIST.	This is 
     the same reason that BASE and HEIGHT did not show up once they became 
     equations.  To the calculator, BASE=6 is a defined equation; BASE 
     is not allowed to be anything other than 6 according to this equation. 
 
     The answer to what Mr. Sultenfuss was attempting is to use the store 
     key.    6 -> BASE:8 -> HEIGHT:DIST would set these variables to the 
     value 6 and 8 and allow the value 10 for DIST to be displayed without 
     changing the status of BASE and HEIGHT as real number variables. 
 
     We appreciate your questions and hope this discussion is helpful. 
     You may send comments or questions through GRAPH-TI or by email 
     to the address below. 
 
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