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Re: what is my error?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100465] Re: [mg100438] what is my error?
  • From: "David Park" <djmpark at comcast.net>
  • Date: Fri, 5 Jun 2009 03:00:03 -0400 (EDT)
  • References: <18096534.1244101248730.JavaMail.root@n11>

Richard,

eqns = {mr + mp == 13, fr + fp == 19, mr + fr == 15, mp + fp == 17};

These equations are not all independent so if we use NSolve or Solve or
Reduce we do not obtain a complete solution.

Reduce[eqns]
mp == 13 - mr && fr == 15 - mr && fp == 4 + mr

We can see this by setting up the matrix and rhs to use LinearSolve:

m = ( {
    {1, 1, 0, 0},
    {0, 0, 1, 1},
    {1, 0, 1, 0},
    {0, 1, 0, 1}
   } );
b = {13, 19, 15, 17};
Det[m]
0

But, LinearSolve will obtain a solution as long as the rhs is linearly
dependent in the same way as the rows of the matrix are.

lsols0 = LinearSolve[mat, {13, 19, 15, 17}]
lsols1 = {mr, mp, fr, fp} -> lsols0 // Thread	
{-4, 17, 19, 0}
{mr -> -4, mp -> 17, fr -> 19, fp -> 0}

eqns /. lsols1
{True, True, True, True}


David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/  


From: Richard Palmer [mailto:rhpalmer at gmail.com] 


eqns = {mr + mp == 13, fr + fp == 19, mr + fr == 15, mp + fp ==
17}NSolve[eqns,
{mr, mp, fr, fp}]

gives {{mr->-4.+1. fp,mp->17.-1. fp,fr->19.-1. fp}}

The solution is  {mr->8, mp->5, fr->7, fp->12} if mr,mp,fr,fp are greater
than zero

-- 
Richard Palmer



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