HP 50g Graphing Calculator - Manual (Page 113)

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Pressing the soft menu key corresponding to p1 will provide an error message
(try L
@@@p1@@ `):
Note: By pressing @@@p1@@ ` we are trying to activate (run) the p1 program.
However, this program expects a numerical input. Try the following exercise:
$
@@@p1@ „Ü5`. The result is:
The program has the following structure: << → r 'π*r^2' >>
The « » symbols indicate a program in User RPL language. The characters → r
indicate that an input, to be referred to as r, is to be provided to the program.
The action from the program is to take that value of r and evaluate the
algebraic '
π*r^2'. In the example shown above, r took the value of 5, and thus
the value of πr 2
= π⋅25 is returned. This program, therefore, calculates the area
of a circle given its radius r.
RPN mode
In RPN mode, you only need to press the corresponding soft menu key label to
get the contents of a numerical or algebraic variable. For the case under
consideration, we can try peeking into the variables z1, R, Q, A12,
α, and A,
created above, as follows: J @@z1@@ @@@R@@ @@@Q@@ @@A12@@