Decimal point before an operation

[Jim Yohe]

Looking at the HP 50g User's Guide on page 1-14 it shows that you can input an expression directly without using the equation writer. While in the algebraic operating mode, with every entry of a whole number they also show a key-press of a decimal point before an operation (ie, "x" "+" etc..). When in the equation writer they didn't do this and so it's foreign to me, as I've never had to do that with other HP and TI calculators I've owned.

Why was it necessary to add a decimal point before an operation?

[Chris Randle (UK)]

Hi Jim,

It's not a decimal point before the operator, it's a decimal point after the number. It's a way of overriding the current exact/approximate mode and explicitly stating that you want an approximate result (a numeric answer) rather than an exact result with integers.

You can toggle implicit exact/approximate mode by holding down right-shift and pressing enter.

If you're in algebraic mode (ALG displayed in the header) and in exact mode (= displayed in the header), try these:

1 / 2 [ENTER] -> '1/2'

1. / 2. [ENTER] -> '.5'

In the last example, you could have left the decimal point off either the 1 or the 2 and you'd have got the same result.

If you had tried the example from page 1-14 without the trailing decimal points, the calculator would have produced a non-approximate result, but the EXP() function must produce an approximate result, so the calculate instead asks you to confirm a switch to approximate mode.

If the calculator is in exact mode and you want an approximate answer, you can use (right-shift) ->NUM

1 / 3 [ENTER] -> '1/3' [->NUM] -> '.333333333333'

Edited: 29 Jan 2011, 3:51 p.m.

[Jim Yohe]

Thank you Chris. I am just reading the PDF of the manual and wasn't aware of approximate vs. exact modes. At least not yet. Still it's a bit confusing since this is a new concept to me with respect to how this calculator has been programmed to work.

[Chris Randle (UK)]

It's a great feature of the calculator, and well-implemented, IMO. In case you're not familiar with HP tri-glyphs, "\v/" represents the square root symbol, "\->" a right-arrow, and "\pi" the symbol for pi.

In exact mode,

\v/4 -> 2

because there's an exact decimal representation of the square root of four. But, because the square roots of two and 20 are irrational,

\v/2 -> \v/2

\v/20 -> 2*\v/5

Keeping the square roots there in case a later squaring operation is used. If you knew in advance that you wanted an approximate decimal fraction for the square root of two, you could type:

\v/2. -> 1.41421356237

to convert back,

\->Q\pi(ANS(1)) -> \v/2

\->Q\pi is available from the catalog menu (CAT)

The example on page 1-14 and the examples I've used assume the calculator is in algebraic mode. I prefer RPN mode, and Tim Wessman has produced an additional guide: "HP 50g Graphic Calculator Quick Start Guide" with all examples in RPN mode. If you haven't seen it, you can download a copy here:

HP 50g Graphing Calculator Quick Start Guide

Also, if you haven't seen a copy of the Advanced User's Reference Manual, which is definitely worth a look, you can get yourself a copy from here:

HP 50g/49g+/48gII graphing calculator advanced user's reference manual

[Jim Yohe]

Thanks again. I wanted to point out that the second link is broken with a message of, "The page you requested can not be found".

Does this happen to now be the "Advanced_Users_Ref_Manual.zip" file you can download from here?:

http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?lang=en&cc=us&contentType=SupportManual&docIndexId=64255&prodTypeId=215348&prodSeriesId=3235173&lang=en&cc=us

Edited: 29 Jan 2011, 8:14 p.m.

[Chris Randle (UK)]

Sorry about the broken link. Could have sworn it worked when I tried it. But yes, that zip file is the one I meant. The AUM's a great work.