HP 39gs_40gs_Mastering The Graphing Calculator_English_E_F2224-90010

Pitfalls in Fraction mode
The
Fraction setting is thus far more powerful than most calculators but
can require that you understand what is happening. It should also be
clear now why a special fraction button was not provided: the ‘fractions’
are never actually stored or manipulated as fractions at all!
As you can see above right, a setting of
Fraction 4 produces a strange (but actually correct) result for
0.666, while adding one more 6 (to take the decimal beyond 4 d.p.) will give the desired result of
2/3. In
other words, so long as you understand the approach taken by the hp 39gs and hp 40gs it is capable of
producing results which are closer to what was probably intended by the user in entering 0.66666.
You may have noticed that all the results so far have been improper
fractions. For example the first calculation shown right gives the answer
as 22/15 rather than
7
1
15
. The fraction setting of Mixed Fraction is
essentially the same but answers are given as mixed fractions instead of
improper fractions, as shown.
If you want to use the
Fraction setting to convert decimals to fractions, here are some tips…
• if converting a recurring decimal to a fraction, then make sure
you include at least one more digit in the decimal than the
setting of
Fraction in MODES. As you can see right, failing to
include enough decimal places does not produce the desired
result.
• if you are converting an exact decimal to a fraction, then set a
Fraction n value of at least one more than the number of
decimal places in the value entered. Both examples in the third
screen shot to the right were done at
Fraction 6.
Not understanding the significance of the setting of
Fraction can
produce some unfortunate effects. For example, at Fraction
2, the value
of
123.456 becomes 123, with the 0.456 dropped entirely.
An example of this is shown right. If you use a setting of only
Fraction 2
to perform the calculation shown, you will find to your amazement that
1/3 + 4/5 = 8/7 , whereas using Fraction 6 gives the correct answer.
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