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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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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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