Solution of a system with a parameter in the solution
08-12-2017, 06:00 PM
Post: #1
 mazzeodalessio Junior Member Posts: 1 Joined: Aug 2017
Solution of a system with a parameter in the solution
Hello guys. I have a big problem, which I can not solve. I bought a HP calculator, namely the HP Prime.
I took it especially because I have to solve several systems during a complex electronics exam, doing it all by hand would be crazy unfortunately.

I expose you the problem, I have in my system 4 equations:

Vx = Vi - I1*3
Vx = Vg + I2*5
I1 = I2 + I3
I3 = (Vx - 2Vg)/10

Now, I should calculate a output Vu = 5 - 250 *I3.

Just because I want the Vu result as a function of Vi.

By doing the hand calculation I come (right) that:

Vu = 30.66-13.16 * Vi

The problem is that I do not know how to set this calculation on the calculator, I tried in all ways using the "Solve" function but nothing. Can you give me a hand? You would solve a big problem!
08-20-2017, 11:46 AM
Post: #2
 Aries Member Posts: 153 Joined: Oct 2014
RE: Solution of a system with a parameter in the solution
The system is under-determined, to be honest I don't know how to find Vu in function of Vi (from the perspective of the calculator).
Best,

Aries ;-)
08-20-2017, 08:32 PM (This post was last modified: 08-20-2017 10:21 PM by Gilles59.)
Post: #3
 Gilles59 Member Posts: 136 Joined: Jan 2017
RE: Solution of a system with a parameter in the solution
(08-12-2017 06:00 PM)mazzeodalessio Wrote:  Hello guys. I have a big problem, which I can not solve. I bought a HP calculator, namely the HP Prime.
I took it especially because I have to solve several systems during a complex electronics exam, doing it all by hand would be crazy unfortunately.
(...)

I dont undestand how you get your result. If I'm not wrong you have a system of 5 equations (include 'Vu=5-250*I3' ) and 7 unknows (or 5 unknows and 2 parameters and you need 5 unknows and 1 parameter only to get 'Vu' in function of only 'Vi')

With the HP50G (exact mode) you can do :

Code:
 ['Vx=Vi-3*I1'  'Vx=Vg+I2*5'  'I1=I2+I3'  'I3=(Vx-2*Vg)/10'  'Vu=5-250*I3']  ['Vu' 'I1' 'I2' 'I3' 'Vx' ]  SOLVE (or LINSOLVE)

And you get :
Vu=(650*Vg-(250*Vi-95))/19

I guess you can use LINSOLVE on the PRIME CAS do do this.

Are you sure that there is no typo error in your message, or that you have not forgotten an equation ?

EDIT : I get your result if I assum Vg=3/4

EDIT 2 : With the Prime, assuming Vg=0.75, CAS mode :

Code:
linsolve( [Vx=Vi-3*I1,   Vx=Vg+I2*5,  I1=I2+I3,   I3=(Vx-2*Vg)/10,   Vu=5-250*I3, Vg=0.75],  [Vu,I1,I2,I3,Vx,Vg]) Ans(1) Collect(Ans)
08-25-2017, 07:57 AM (This post was last modified: 08-25-2017 08:02 AM by math7.)
Post: #4
 math7 Member Posts: 181 Joined: Feb 2017
RE: Solution of a system with a parameter in the solution
In fact, there must be at least one or two more equations in your approach. If I am not mistaken, you are solving a system of equations with mesh method in an electronic circuit. There is also the method of nodes, right? When solved by either method, to obtain a numerical result, the number of equations must be equal to the number of unknowns.
And remember that apart from the solve command of the calculator you can also now use the Equation Library application of our friend Han that is available here http://www.hpmuseum.org/forum/thread-772...on+library
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