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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