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Re: Wrong Answer

  • To: mathgroup at smc.vnet.net
  • Subject: [mg130667] Re: Wrong Answer
  • From: Daniel <dosadchy at its.jnj.com>
  • Date: Wed, 1 May 2013 03:36:54 -0400 (EDT)
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  • Delivered-to: l-mathgroup@wolfram.com
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> 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]
> 

I use Mathematica 8 and Solve[p,t]/.r returns "Infinite expression 1/Sqrt[0.]..." so obviously there is a division by 0 problem, which could be solved with Limit[].

So "Solve[p, t] /. r" fails, as well as "Solve[p, t] /. Most[r] /. Last[r]"

but,

Limit[t /. Solve[p, t] /. Most[r], Last[r]]

gives the same answer as 

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]



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