Re: Wrong Answer

*To*: mathgroup at smc.vnet.net*Subject*: [mg130637] Re: Wrong Answer*From*: Murray Eisenberg <murray at math.umass.edu>*Date*: Sun, 28 Apr 2013 00:59:19 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net*References*: <20130427030938.151746A1A@smc.vnet.net>

If I use exact rationals for the parameters -- r := {px -> 1/2, py -> 1/2, vx -> 1/2, vy -> 1/2} -- then Solve[p,t]/.r gives division by 0 errors. That's with Mathematica 9.0.1 under OS X (10.8.3). On Apr 26, 2013, at 11:09 PM, christopher arthur <chris.arthur1 at gmail.com> wrote: > Hello, > > I've discovered that Solve[] gives me the wrong answer for some > polynomial equations. For example, consider the following equation, > which should have exactly one positive, real answer for t. > > p:=(px + t vx)^4 + (py + t vy)^4==1 > r:={px->0.5,py->0.5,vx->0.5,vy->0.5} > NSolve[p/.r,t] > > NSolve gives the right answers if I substitute for px,py,vx and vy > before solving. On the other hand, if I use Solve[] this way: > > Solve[p,t]/.r > > I get a different answer, and it's not correct. > Does Solve[] just malfunction with quartic polynomials or is there > something else happening? > > Chris > > mathematica 5.2 > > Solve[p] --- Murray Eisenberg murray at math.umass.edu Mathematics & Statistics Dept. Lederle Graduate Research Tower phone 413 549-1020 (H) University of Massachusetts 413 545-2838 (W) 710 North Pleasant Street fax 413 545-1801 Amherst, MA 01003-9305

**References**:**Wrong Answer***From:*christopher arthur <chris.arthur1@gmail.com>