Re: Simple questions about Mathematica

*To*: mathgroup at smc.vnet.net*Subject*: [mg31891] Re: [mg31888] Simple questions about Mathematica*From*: BobHanlon at aol.com*Date*: Mon, 10 Dec 2001 06:14:30 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

In a message dated 12/9/01 7:11:31 AM, stevebg at adelphia.net writes: >1. Suppose I solve some equations and get a complicated >expression containing multiple identical elements such as >sqrt(a^2+b^2). To make the expression smaller, clearer, and >easier to deal with, I want to substitute say R=sqrt(a^2+b^2) >everywhere in the main expression and have R available as >a symbol from then on. I see nothing in Help or anywhere >else about how to do this. But it's such a standard thing to >do that there must be an easy answer. > >2. I'm not sure how a "batch" file is suppposed to be >prepared and fed to Mathematica. That is, I want to >prepare a bunch of operations and variables in advance >and feed it in, preferably having mathematica give me >line-by-line output as it would if I were typing each line >in one by one. > 1. Use substitution rules: Clear[a, b, R]; useR = (Sqrt[a^2+b^2] :> R); removeR = (R :> Sqrt[a^2+b^2]); expr = (Sqrt[a^2+b^2]+c)/(Sqrt[a^2+b^2]-c); expR = (expr /. useR) (c + R)/(-c + R) (expR /. removeR) == expr True 2. In the Master index, look under 'Batch files" Bob Hanlon Chantilly, VA USA