The presented exercise basically includes different sequences of scholar activity like eventually collecting information about credits, setting up assumptions, creating the GeoGebra applet, observing dynamics and coherences, proving the observations under the formulated assumptions, revealing misbehaviour of the model. The use of the computer
is not restricted to creating an applet or observing dynamics, its application is proper
for almost every stage of the modelling cycle (see [4], p. 372), in particular: collecting
information in the internet and proving (e.g. using the CAS).
There are various application possibilities of this exercise in class: The teacher implements the whole exercise in class, including all modelling cycles as presented. Or, only
one modelling cycle is discussed in class. Or, only one credit is modelled in class e.g.
foreign currency loan. This exercise can be shortened (pupils' activity), in particular:
The teacher sets up such GeoGebra applets, pupils work on prepared guiding questions.
Or, there is no mathematical reasoning for the heuristic observations in class.
Wittmann suggests in the context of the "genetic principle" to change the point of view
(see [5], p. 148). As well one can do this in this exercise: We fix the lent term to 30
years, and let the instalments vary. Then one receives the applet of figure 5: