Re: How to discard negative solutions

*To*: mathgroup at smc.vnet.net*Subject*: [mg129312] Re: How to discard negative solutions*From*: Bob Hanlon <hanlonr357 at gmail.com>*Date*: Tue, 1 Jan 2013 20:04:11 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <20130101004551.3FFB56895@smc.vnet.net>

?$Assumptions $Assumptions is the default setting for the Assumptions option used in such functions as Simplify, Refine, and Integrate. >> $Assumptions is not used by Solve directly. Imbed the $Assumptions as constraints in the Solve or use Select or Cases or DeleteCases along with Simplify/FullSimplify to eliminate undesired results. $Assumptions={a>0,b>0,c>0}; solveSolution={b->a,c->a,c->-a-b}; Select[solveSolution,Simplify[Positive[#[[-1]]]]&] {b->a,c->a} Bob Hanlon On Mon, Dec 31, 2012 at 7:45 PM, Dr. Robert Kragler <kragler at hs-weingarten.de> wrote: > Hi, > from a geometric problem I obtain with $Assumptions = {a > 0, b > 0, c > 0} > using Solve a series of solutions such as > {b -> a, c -> a} but also {c -> -a - b}. > The latter one should obviously be discarded because it will be negative > according to the assumptions (a>0, b>0) above. > How can these negative solutions being excluded by Mathematica V8 ? Positive[] > and NonNegative[] do not help because quantities which are not NumericQ remain > unevaluated. Help is appreciated. > > Robert Kragler >