HP 40gs HP 39gs_40gs_Mastering The Graphing Calculator_English_E_F2224-90010.p - Page 79

Nice scales in the PLOT-TABLE view A time when 'nice' scales are more important is when you use the PlotTable option in the VIEWS menu. If you use the default axes you will 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 PlotTable 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 F1(X) = X2(X+1) as X2*(X+1) but will not understand F(X)=X(X+1). When used in either 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 S=A(1-R^N)/(1-R) then you will see a similar 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 SIN(....) and COS(....) and ROUND(....) work with brackets. When the calculator encounters X(X+1) it interprets this as asking it to evaluate a function called X(....) at the value X+1. Since there is no such function it returns the error message 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. 79