Re: Differencing two equations

• To: mathgroup at smc.vnet.net
• Subject: [mg129760] Re: Differencing two equations
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Mon, 11 Feb 2013 04:37:32 -0500 (EST)
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• References: <20130210082414.C3A5F6937@smc.vnet.net>

```eqnOperate[eqn1_, eqn2_, f_] :=
Equal @@ f @@ {List @@ eqn1, List @@ eqn2};

eqnOperate[a == r, b == s, Subtract]

a - b == r - s

eqnOperate[a == r, b == s, Plus]

a + b == r + s

eqnOperate[a == r, b == s, Times]

a b == r s

eqnOperate[a == r, b == s, Divide]

a/b == r/s

eqnOperate[a == r, b == s, Power[#1, #2] &]

a^b == r^s

Bob Hanlon

On Sun, Feb 10, 2013 at 3:24 AM,  <g.c.b.at.home at gmail.com> wrote:
> I'm brand new to Mathematica, so I apologize for the naive questions...
>
> 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.
>
> The Core Language documentation makes a big point of how everything is basically a list with different heads.  In this case, what I'm trying to do would work if it were treated as a list ({a,b}-{r,s} returns {a-b,r-s}) but doesn't work under Equal.
>
> Thanks for any suggestions.
>

```

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