HP50g DESOLVE Help



#2

On page 14-3 of the 50G Users manual are some commands to use the DESOLVE. The second example is entering:


['d1d1y(t)+5*y(t)=2*COS(t/2)' 'y(0)=6/5' 'd1y(0)=-1/2']

ENTER 'y(t)' ENTER DESOLVE

The solution "should" be in terms of sine and cosine, as is reported in the manual. Mine keeps the solution in terms of the complex expressions e^(a+bi), etc. I can certainly do the rewriting myself, but what possible setting am I missing to have the calculator to convert to sines and cosines?

The modes I am using are: soft menues, RPN, RAD, Complex, 't' var, the usual... But I must be overlooking something.

Any suggestions?

Thanks, all.


#3

Using ROM version 2.09:

1) the variable stored in 'VX' should be 't'

2) the machine should be in radians, real, exact mode: you should see RAD XYZ R = 't' at the screen top

With that done, on my 50g:

['d1d1y(t)+5*y(t)=2*COS(t/2)' 'y(0)=6/5' 'd1y(0)=-1/2']

'y(t)'

DESOLVE returns, and after one minor simplification:

'COS(t*1/2)*8/19+COS(t*SQRT(5))*74/75-SQRT(5)/10*SIN(t*SQRT(5))'

Not bad for a handheld, Maple's closed form for this ODE is quite similar.


#4

Of course, flag settings are probably to blame.

One thing you could do is completely reset your 50g, or post your complete flag settings to let us see what flags are set.

#5

Hmmm. No go. So here's a simpler one that I tried...

['d1d1y(T)+y(T)=0' 'y(0)=2' 'd1y(0)=0']
'y(T)'

DESOLVE returns (after a simplification)

      (e^(iT))^2 + 1
y(T)=---------------
e^(iT)

It would be nice if it just gave back 2cos(T). :(

The obvious modes shown on the screen are:
RAD XYZ BIN C='T'

In the CAS MODES I have Complex checked, and Simp Non-Rational checked.

The system flags that are checked are {27, 34, 40, 56, 57, 68, 76, 82, 90, 103, 117, 119}

My version number is #2.09

Any more clues????

Thanks.

Edited: 24 Apr 2007, 11:52 p.m.


#6

Change COMPLEX to REAL mode. The shortcut key method is press and hold LEFT SHIFT, then press and release the TOOL key. This is a toggle between COMPLEX to/from REAL mode. You should see the 'C' change to an 'R' and vice versa when this key sequence is done.

At the top of the screen, you should see RAD XYZ DEC R='T'

Otherwise, solve your difeqs with Maple or Matlab...


#7

Quote:
At the top of the screen, you should see RAD XYZ DEC R='T'

Otherwise, solve your difeqs with Maple or Matlab...


Ahhh. The Real mode makes sense now. Thanks. Normally I DO use Mathematica, or occassionally Maple, (when not not doing them on paper) but it's hard to carry my desktop computer around with me. ;) Also, I need to know what my students are capable of doing with their magic black boxes!!!

Thanks again.

CHUCK

Edited: 25 Apr 2007, 12:32 p.m.


#8

Your're welcome! Oh, you're students are so lucky to be able to use such marvels as an HP50!

#9

I see you got this "Complex" problem around to a "Real" solution. My 50, even with ROM still degraded to 2.08, wants to use complex equivalents when the DE lends itself to complex solutions. (I carried a K & E "Deci-lon," scabbard and all, when I was a student [ University of New Orleans, 1967 and Tulane University.]

I agree with you that Chuck's students have a wonderful, portable solution to quicker understanding and modeling of engineering challenges.

Regards,

Ron

Edited: 26 Apr 2007, 12:46 a.m.

#10

Where do you teach, Chuck? And at HS or college/university?

Rich


#11

Hi Rich. I'm at a two-year college in Washington state. We're covering harmonic motion, damped, undamped, and forced, using a number of tools. Several kids have the HP50G wihich is blowing the socks off the TI's. I'm amazed at some of the things it does. When I did this in college as a student, "all" I had was the HP-11C. But that was about 10-years post-sliderule times, so I thought I was in fat city.


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