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Posts: 1
Member Since: ‎12-07-2013
Message 1 of 2 (8,418 Views)

HP Prime calculating bug

[ Edited ]

Hi, I'll get right to the issue i'm having with my HP Prime calculator.  Recently during an exam where i was calculating the moments of inertia of certain bodies i encountered what i can only describe as a bug in the software because I can't figure any other reason why it would kick out the answer it did for such a seemingly simple function.  After I had done all the laboreous work of simplifying the differential equations I just needed to add some simple fractions.  As i have been operating this calculator for several weeks now, the only way i can get it to show a fraction in exact form is to type (number) / (number) and it shows it in exact form.  In my case and for this particular calcuation I input (3/20) - (1/7) "enter".  the answer Prime gave me was noticably unsimplified given that i needed it in simplified form.  although the numerical, non-exact value was almost correct (.00714285714309 instead of the correct value of .00714285714286), generally Prime kicks out a simplified value.  What it gave me was 214500215/30030030099 instead of 1/140.  i'm just curious if i need to do something different to get a simplified/correct answer on the first go rather than having to do a work-around.


Thanks in advance

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Posts: 6
Member Since: ‎11-11-2013
Message 2 of 2 (8,402 Views)

Re: HP Prime calculating bug

If you want an exact and simplified answer on the first go, do your calculation in CAS view rather than Home view. Try this experiment.


1. Go to CAS view, and enter your calculation (3/20)-(1/7). The Prime will respond with 1/140, the exact answer. Now press the a b/c key. The Prime will respond with 0.00714285714286, the approximate decimal equivalent. Keep pressing the a b/c key, and the Prime will toggle between the two answers.


2. Now try the same steps in Home view. The Prime will toggle between 0.007142857143 and 214500215/30030030099.


Remember, the true decimal value for 1/140 is an infinite repeating number. Since every calculator can store only a finite number of digits, it must truncate the value after a set number of digits, and this introduces a small error. The big fraction you got in Home view is not wrong, and it's certainly not a bug. It's just that Home view is not designed to handle exact values. The CAS, on the other hand, is designed to handle exact values, so when you tell it to show an approximate answer, its decimal form will be closer to the exact value, namely, 1/140.


The moral of the story is, use CAS view when you want exact values and Home view when you want approximate values. I tell my students to use fractions whenever possible (since those are usually exact) rather than converting them to decimal values. It's far easier to use 1/3 than 0.33333....[infinite number of 3s go here].


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