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Wrong Answer


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]



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