Re: Differencing two equations

• To: mathgroup at smc.vnet.net
• Subject: [mg131479] Re: Differencing two equations
• From: nma%12000.org at gtempaccount.com
• Date: Sun, 4 Aug 2013 05:16:44 -0400 (EDT)
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• References: <20130210082414.C3A5F6937@smc.vnet.net> <kfae39\$m53\$1@smc.vnet.net>

```On Monday, February 11, 2013 3:36:41 AM UTC-6, Bob Hanlon wrote:
> eqnOperate[eqn1_, eqn2_, f_] :=
>
>   Equal @@ f @@ {List @@ eqn1, List @@ eqn2};
>
> eqnOperate[a == r, b == s, Subtract]
>
>
> a - b == r - s
>

Hello;

Do you know why this case of subtracting 2 equations does not produce the expected result using your function above?

------------------------------

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

eqn1 = 8*(-f[-h + x] + f[h + x] == 2*h*Derivative[1][f][x]);
eqn2 = -f[-2*h + x] + f[2*h + x] ==
4*h*Derivative[1][f][x] + (8/3)*h^3*Derivative[3][f][x] +
(8/15)*h^5*Derivative[5][f][x];

eqnOperate[eqn1, eqn2, Subtract]
-------------------------------------

Out[4]= 8 + f[-2*h + x] -
f[2*h + x] == (-f[-h + x] + f[h + x] == 2*h*Derivative[1][f][x]) -
4*h*Derivative[1][f][x] -
(8/3)*h^3*Derivative[3][f][x] - (8/15)*h^5*Derivative[5][f][x]

You can see the result has too many '==' inside it.

thanks,
--Nasser

```

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