Re: Replacement
- To: mathgroup at smc.vnet.net
- Subject: [mg4531] Re: [mg4433] Replacement
- From: Allan Hayes <hay at haystack.demon.co.uk>
- Date: Wed, 7 Aug 1996 04:17:39 -0400
- Sender: owner-wri-mathgroup at wolfram.com
ges at vsnhdd.cern.ch (Sergej Gerassimov) [mg4433] Replacement writes > I have something like > x/Sqrt[a^2+b^2]+Sqrt[a^2+b^2]/y > and I'd like to substitute Sqrt[a^2+b^2] by r. > > x/Sqrt[a^2+b^2]+Sqrt[a^2+b^2]/y //. Sqrt[a^2+b^2]->r > gives: > > x r > ------------- + - > Sqrt[a^2+b^2] y > > (replace one only in the numerator) > How to replace Sqrt[a^2+b^2] by r everywhere in the expression? Serge: Two suggestions x/Sqrt[a^2+b^2]+Sqrt[a^2+b^2]/y /. {Power[(a^2+b^2), p:1/2|-1/2]->r^Sign[p]} x r - + - r y x/Sqrt[a^2+b^2]+Sqrt[a^2+b^2]/y /. {Sqrt[a^2+b^2]->r,1/Sqrt[a^2+b^2] ->1/r} x r - + - r y The reason is that what Mma sees is FullForm[x/Sqrt[a^2+b^2]+Sqrt[a^2+b^2]/y] Plus[ Times[Power[Plus[Power[a, 2], Power[b, 2]], Rational[-1, 2]], x], Times[ Power[Plus[Power[a, 2], Power[b, 2]], Rational[1, 2]], Power[y, -1] ] ] The Sqrt has gone. Allan Hayes hay at haystack.demon.co.uk ==== [MESSAGE SEPARATOR] ====