HP 39gs Master Manual page 79

Nice scales in the PLOT-TABLE view
A time when 'nice' scales are more important is when you use the Plot-
Table option in the menu. If you use the default axes you will
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find that the dots, and hence the table values are no longer 'nice'
because of the dots consumed by the line down the middle of the screen.
This can be solved by changing the x axis
scale to -6.4 to 6.4, which gives table
values of 0.2.
Using -3.2 to 3.2 is even better since it makes the graph 'square' again,
with both axes proportional. Another good choice of scale for the Plot-
Table view is -8 to 8, giving table values of 0.25. Basically any power
of 2 is a good choice. Again, adding or subtracting a constant from
each end of the axes will produce a graph where the y axis is not
centred.
Use of brackets in functions
One problem commonly encountered by new users is misinterpretation of
brackets. The hp calculator will correctly interpret as
F1(X) = X (X+1)
2
but will not understand . When used in either
X *(X+1) F(X)=X(X+1)
2
Function or Solve, it will result in the error message of "Invalid User
Function".
Similarly if you want to use the sum to n terms formula for a GP in the
Solve aplet and enter it as then you will see a similar
S=A(1-R^N)/(1-R)
message unless you change it to read .
S=A*(1-R^)/(1-R)
The reason for this apparent 'error' is that all of the built-in functions such as and and
SIN(....) COS(....)
work with brackets. When the calculator encounters it interprets this as asking it to
ROUND(....) X(X+1)
evaluate a function called at the value . Since there is no such function it returns the error message
X(....) X+1
that you are trying to use a function that is unknown.
The solution is simple: just remember to put the * sign in when you use letters immediately before a bracket.
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