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Re: simultaneous equations

  • To: mathgroup at
  • Subject: [mg2269] Re: [mg2232] simultaneous equations
  • From: Lou Talman <me at>
  • Date: Thu, 19 Oct 1995 01:34:22 -0400

There is a syntax error in the third equation of your system.

You should have had "r7880 (q78 +q80]^2", and not "r7880[q78 +q80]^2".

--Lou Talman

Begin forwarded message:

>From: heath andersen <ha20 at>
>To: mathgroup at
> Subject: [mg2232] simultaneous equations

i am having trouble solving this set of simultaneous equations. it is
probably my maths that is letting me down here, or lack of Mma use, or both,
but, i can not work it out and have not had much luck asking round. 

In case u wanted to know i am trying to work out the volumetric flow rates
for a dust extraction system,  using an electrical analogy, where 'p' is the
pressure(Pa), 'r' the resistance to flow and 'q' is the volumetric flow
rate. the numbers ar the node opoints of the system, for example, 'p78',  is
the pressure at node 78, and 'p7880' is the node where 'p78' and 'p80' come
together abd so on... 

also, [q7880]^2 = [q78 + q80]^2

Here is exactly what i typed in and what Mma gave me back.

p78 = p80 = p81 = 101300

p788081 = 100100.6

r78 = r80 = 220

r81 = 219.5

r7880 = 54.86

Solve[{	p78 - p7880 == q78^2 * r78, 

	p80 - p7880 == q80^2 * r80,
	p7880 - p788081 == r7880[q78 +q80]^2,
	p81 - p788081 == q81^2 * r81}, 

	{q78, q80, p7880, q81}]  


   The equations appear to involve
     transcendental functions of the
     variables in an essentially
     non-algebraic way.
Solve[{101300 - p7880 == 220 q78 , 


  101300 - p7880 == 220 q80 , 


  -100101. + p7880 == 54.86[q78 + q80] , 


  1199.4 == 219.5 q81 }, 


 {q78, q80, p7880, q81}]

                                             university of wollongong
                                             nsw australia  

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