• 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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```> Hello,
>
> I've discovered that Solve[] gives me the wrong
> 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]]