HP 20b: +/- and ANS (or CHS and LASTx)

## HP 20b Using the RPN stack to solve problems efficiently

This solution works on all RPN-calculators I know but gives -0.91 on HP 20b instead. The reason is that +/- copies register X to ANS. Thus the 2nd ANS returns 1/x instead of x.

Microchallenge: Can you help HP and find the correct sequence?

Here's the link to get all HP 20b Training Modules.

Prologue

Despite the fact the 20b has a RPN mode, it has a strange "new" behaviour, contrary to the also new 35s and the dozens of classic hp RPN calculators:

ENTER after numeric keystrokes doesn't copy X to Y but it does only finalize the input buffer (it isn't X) and copy it to the 1st stack register in 20s.

Incredible, but it works like the high end RPL models. I don't know why they choose to change this important detail of RPN input paradigma...

Possible solution

3,4567 INPUT

INPUT

shift SQR

2 ANS

shift LN

x

+

X<>Y

shift 1/x

-

20 keystrokes and right answer: 4,05

You've cheated a little since ANS is shifted. But still we can do it with 20 keystrokes:

```3.4567
shift SQRT
shift ANS
INPUT
shift LN
2
*
)x
shift 1/x
-
+
```

 Possibly Related Threads… Thread Author Replies Views Last Post HP-80 CHS Exponent Curiosity Max Stone 4 1,800 10-22-2013, 02:39 PM Last Post: Max Stone HP's thinking behind the 20b/30b? John Ioannidis 3 1,588 09-07-2013, 10:21 AM Last Post: Tim Wessman 20b, 30b not in HP's web store Eric Smith 3 1,413 02-08-2013, 11:52 AM Last Post: Walter B A non 20b/30b 34S Matt Agajanian 22 4,998 08-20-2012, 01:06 AM Last Post: Reth [WP-34s] IR conversion of 20b Alexander Oestert 3 1,495 05-18-2012, 05:38 PM Last Post: Harald LASTx for the 35 Matt Agajanian 3 1,322 03-23-2012, 03:29 AM Last Post: Massimo Gnerucci (Italy) WP 34S - INC X and RCL L (LASTx) Gerson W. Barbosa 10 2,434 12-26-2011, 05:33 PM Last Post: Paul Dale Why hp-20b more expensive than 30b? Masaki Adachi 9 2,802 10-14-2011, 12:08 PM Last Post: megarat missing segments on 20b running 34s db (martinez, ca.) 4 1,594 10-05-2011, 01:27 AM Last Post: Walter B Flashing HP 20b for WP 34S Steve Brenton 10 2,582 09-30-2011, 09:03 AM Last Post: Marcus von Cube, Germany

Forum Jump: