HP 25S scientific calculator, Equation function.

[joecolgan]

how do I enter this into the equation function of my 35s Calculator?

P=(DxI/12)/(1-(1+I/12)^-(T*12) ? I am struggling with the parentheses; the formula works in Excel but not in the calculator; it is the formula for working out mortgage payments on a P&I basis.

Many thanks if you can help,

Joe.

[BartdB]

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joecolgan wrote:

how do I enter this into the equation function of my 35s Calculator?

P=(DxI/12)/(1-(1+I/12)^-(T*12) ? I am struggling with the parentheses; the formula works in Excel but not in the calculator; it is the formula for working out mortgage payments on a P&I basis.

Many thanks if you can help,

Joe.

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For the negative in the power part [ i.e. -(T*12) ], did you use the +/- ? Using a normal minus without a preceding number or variable, produces an error.

By the way, your formula as shown above is missing a closing parenthesis, but this should not matter as the 35s automatically adds it.

Once entered it should look something like this:

Solving P for the following data:
D = 12 000
I = 12% = 0.12
T = 5 years

P = 266.93

Same as this formula in Excel and the finance formulas on the 50G.

Regards,
BartdB

Edit: changed a "/" to a divide sign

[joecolgan]

Hi Bartb, thanks for that, yes I did use the +/- rather than the '-' and I've checked and re entered and when I try to execute the formula by EQN, Solve, P, I get either a syntax error or a Select FN message sometimes one, sometimes the other.  I am begining to think it might be faulty.

Regards,

Joe.

[BartdB]

"SELECT FN" usually appears if SOLVE is used without entering the EQN menu first. After pressing EQN, make sure that the required function is displayed on the bottom line before SOLVE.

It could be that the batteries are getting low. Replace both with fresh ones, making sure the "+" side is visible for both batteries. (Remove and replace one at a time to save the memory).

You can also try a reset, but this may loose the memory and all saved programs. See page A-4 of the manual ("Testing Calculator Operation") for different resets.

Regards,

BartdB

[BartdB]

Hi,

If you are interested in doing financial calculations on the 35s, then perhaps the following will be of interest to you:

HP-35S Multiple cashflow / NPV / IRR / NFV / NUS program and

Accurate TVM for HP 35s

(please ignore any reference to checksums and length as the 35s computes them incorrectly).

Other stuff:

HP-35s learning modules

Regards,

BartdB

[joecolgan]

Hi Bartb, thanks for the further advice. The FN solution was spot, I didn't have the equation ready to go at the right line. 

I've solved it; the problem was that the calculator takes the 'I' input as a number not a percentage; I tried to enter it as a percentage in the equation but if I put the I in the equation and then press the % key I get a strange input which is %(,) and it is clearly awaiting a prompt. Logically (or so I thought) i put the I in the brackets but  however I do it, with the comma, or without it before the parentheses or after, I got a syntax errror and there is little or no guidance as to how this input shold work in the manual.  After much head scratching I realised I could just convert it to an effective percentage by dividing by 100 so I re entered the formula;

P=D*I/12/100/(1-(1+I/12/100)^ -(T*12)) 

and it works perfectly. 

Strange thing is, I don't think before I accidentally deleted it I had that / 100 in the formula but I guess I must have as the calculator doesn't know that I is percentage figure.

Cheers for the help anyway.

Regards,

Joe. 

[BartdB]

Glad to hear you have got it working.

The % input on HP scientific calculators is not the same as on some of the finacial ones. Indeed on calculators like the 10Bii & 17Bii, on these pressing the % key during input will convert from percentage number to decimal.

The scientifics will (in RPN anyway) take the given percentage of the value in the Y-register. (e.g. 500 ENTER 5 % will =25 i.e. 5% of 500).

I noticed you did not have the "/100" for I, hence in the example in my previous post I indicated entering "I = 12% = 0.12" ;-)

However, the 35s solver will allow you to do calculations while inputting numbers, e.g. say you wish to enter 500 * 10 for D, then when "D?" appears, do: "500 ENTER 10 * R/S", i.e. the solver will only use the number when "R/S" is pressed.

So you can decimalise (or rather normalise) the percentage by doing e.g. "1 ENTER 12 %"

Regards,

Bart