HP 40gs HP 39gs_40gs_Mastering The Graphing Calculator_English_E_F2224-90010.p - Page 80

Problems when evaluating limits, The reason for this is that the calculated value

Page 80 highlights

Problems when evaluating limits In evaluating limits to infinity using substitution, problems can be encountered if values are used which are too large. For example: ex lim x →∞ 2e x + 6 It is possible to gain an idea of the value of this limit by entering the function F1(X)=e^X/(2*e^X+6) into the Function aplet, changing to the NUM view and then trying increasingly large values. As you can see (right) the limit appears to be 0.5, which is correct. It is not the intention here to pretend that this is any sort of thorough mathematical justification but it does provide you with an indication of whether or not you are on the right track. However, if you continue to use larger values then the limit appears to change to 1 (see right). This is obviously not correct, so why is it happening? The reason for this is that the calculated value of ex very quickly passes the upper limit of the capacity of the calculator, which is 10500. When this happens the top and bottom of the fraction become equal (both at a value of 10500 ) instead of the true situation of the bottom being roughly twice size of the top. This error is most likely to happen with limits involving power functions as they will overflow for smaller values of x. The hp 40gs can instead evaluate limits algebraically using the CAS (see page 324). An example is shown right to illustrate the results. 80

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Problems when evaluating limits
In evaluating limits to infinity using substitution, problems can be encountered if values are used which are too
large.
For example:
→∞
2
lim
x
e
x
e
x
+
6
It is possible to gain an idea of the value of this limit by entering the
function
F1(X)=e^X/(2*e^X+6)
into the Function aplet, changing to the
NUM
view and then trying increasingly large values.
As you can see
(right) the limit appears to be 0.5, which is correct.
It is not the intention
here to pretend that this is any sort of thorough mathematical justification
but it does provide you with an indication of whether or not you are on
the right track.
However
, if you continue to use larger values then the limit appears to
change to 1 (see right).
This is obviously not correct, so why is it
happening?
The reason for this is that the calculated value of
e
x
very quickly passes the upper limit of the capacity of the
calculator, which is 10
500
. When this happens the top and bottom of the fraction become equal (both at a
value of 10
500
) instead of the true situation of the bottom being roughly twice size of the top.
This error is
most likely to happen with limits involving power functions as they will overflow for smaller values of x.
The hp 40gs can instead evaluate limits
algebraically using the CAS (see page 324). An
example is shown right to illustrate the results.
80