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Re: Differencing two equations

  • To: mathgroup at smc.vnet.net
  • Subject: [mg129765] Re: Differencing two equations
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Mon, 11 Feb 2013 04:39:13 -0500 (EST)
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On 2/10/13 at 3:24 AM, g.c.b.at.home at gmail.com wrote:

>I'm trying to figure out how to difference two equations.  Basically
>if I have: a==r
>b==s

>I'd like to get: a-b == r-s

>What I'm getting is more like (a==r) - (b==s).  I'm not sure how
>that's a useful result, but is there a function to do what I'm
>looking for?

>A quick search of the archives seem to bring up ways of doing this
>from using transformation rules to swap heads to unlocking the
>Equals operator and hacking its behavior.  I'd like to avoid doing
>that kind of rewiring for a simple operation, and I'd like to keep
>the syntax clean.

Here is one way, but it may not meet your criteria for clean syntax

In[1]:= eq1 = a == r;
eq2 = b == s;

In[3]:= Equal @@ (Subtract @@ List @@@ {eq1, eq2})

Out[3]= a-b==r-s




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