HP 35s Extract Denominator from Fraction
08-11-2014, 04:43 AM
Post: #1
 bcode Junior Member Posts: 8 Joined: Aug 2014
HP 35s Extract Denominator from Fraction
Hi,

Is there any way to extract the denominator (or numerator) from a fraction on the HP 35s? I'm trying to convert mixed fractions to improper fractions. It's a small thing but if it's possible I would appreciate any help.

08-11-2014, 05:29 AM
Post: #2
 Joe Horn Senior Member Posts: 1,797 Joined: Dec 2013
RE: HP 35s Extract Denominator from Fraction
(08-11-2014 04:43 AM)bcode Wrote:  Hi,

Is there any way to extract the denominator (or numerator) from a fraction on the HP 35s? I'm trying to convert mixed fractions to improper fractions. It's a small thing but if it's possible I would appreciate any help.

No, it's not possible, since it's only a display mode, which means that the denominator that's displayed only exists in the display and is not available anywhere for calculations. (If anyone here knows differently, please post the method!)

However, here's a trivial trick that sort of "converts" mixed numbers to improper fractions: just press 1/x, and read the result upside down (as it were).

Example:
left-shift Display: ALL
left-shift pi
right-shift FDISP
--> see 3 16/113
1/x
--> see 113/355 (so the original = 355/113)
Thus we have "converted" 3+16/113 into 355/113.

Unfortunately, this "trick" occasionally fails for various reasons, like here:

7 sqrt
--> 2 2295/3554
1/x
--> 765/2024 (which means 2024/765, but we wanted 9403/3554).

What that happens, you have to do it manually, which is easy since the 35s has a two-line display:
If a+b/c is displayed, then:
Desired Numerator = c*a+b
Desired Denominator = c

<0|ɸ|0>
-Joe-
08-11-2014, 07:37 AM
Post: #3
 bcode Junior Member Posts: 8 Joined: Aug 2014
RE: HP 35s Extract Denominator from Fraction
Oh wow thanks for that tip. And yeah, your example fails because the maximum denominator is 4095, which is understandable.
08-11-2014, 09:06 AM
Post: #4
 walter b On Vacation Posts: 1,957 Joined: Dec 2013
RE: HP 35s Extract Denominator from Fraction
May I recommend a WP 34S for that application? It features DECOMP doing exactly what you were requesting. And it allows for displaying fractions both in proper and improper form.

d:-)
08-11-2014, 04:14 PM
Post: #5
 Joe Horn Senior Member Posts: 1,797 Joined: Dec 2013
RE: HP 35s Extract Denominator from Fraction
(08-11-2014 07:37 AM)bcode Wrote:  And yeah, your example fails because the maximum denominator is 4095, which is understandable.

Exactly. However, sometimes the 1/x trick fails even when the denominator would be 4095 or less, due to roundoff errors, since pressing 1/x doesn't really reciprocate the fraction that's displayed, but instead performs a rounded-off approximation of the reciprocal of a number which itself is already a rounded-off approximation. I just now tried to whip up an example of that, and couldn't find one, but I know I've often seen it in the past, so don't be surprised when it happens.

<0|ɸ|0>
-Joe-
08-12-2014, 03:08 AM
Post: #6
 bcode Junior Member Posts: 8 Joined: Aug 2014
RE: HP 35s Extract Denominator from Fraction
(08-11-2014 09:06 AM)walter b Wrote:  May I recommend a WP 34S for that application? It features DECOMP doing exactly what you were requesting. And it allows for displaying fractions both in proper and improper form.

d:-)

Ah I wish, 75 dollars is quite expensive for a student.
08-12-2014, 03:09 AM
Post: #7
 bcode Junior Member Posts: 8 Joined: Aug 2014
RE: HP 35s Extract Denominator from Fraction
(08-11-2014 04:14 PM)Joe Horn Wrote:
(08-11-2014 07:37 AM)bcode Wrote:  And yeah, your example fails because the maximum denominator is 4095, which is understandable.

Exactly. However, sometimes the 1/x trick fails even when the denominator would be 4095 or less, due to roundoff errors, since pressing 1/x doesn't really reciprocate the fraction that's displayed, but instead performs a rounded-off approximation of the reciprocal of a number which itself is already a rounded-off approximation. I just now tried to whip up an example of that, and couldn't find one, but I know I've often seen it in the past, so don't be surprised when it happens.

Ya well it'll be easy to spot since the numerator of the reciprocal should match the denominator of the original.
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