HP 39gs Master Manual page 161

There are two levels of 'questions' denoted by the keys and
on the screen. An question will be in the main screen ( ­
5 to 5 on each axis), whereas a one can be anywhere in the
larger screen (see right). The setting of also affects the difficulty
of the question. The first is always but you can substitute
another by pressing or .
In mode you must use the arrow keys to change the parameters
'a', 'h' and 'v' until they match the graph shown. The accuracy of your
answer can be checked by pressing the key labeled , which
appears as soon as you begin to change the values. The number of
attempts is monitored and displayed.
An incorrect answer is shown as an 'animated' graph on the screen
when you press , flashing repeatedly between the required graph
and your incorrect guess. This has to be seen to be appreciated - a
screen shot can't do it justice. If your guess and the required graph can't
be shown on the same screen then this animation may not be possible.
If you are unable to find the answer, pressing will display the
correct parameters.
When you are successful, or when you give up, press either or for a new graph, or to
return to the main screen.
If you go to HP's website you can download a worksheet for use with your class. It takes the student through
the process of deducing the effects of each of the coefficients on the shape of the graph, requiring them to
record their answers in writing.
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