Solve vs. NSolve
- To: mathgroup at smc.vnet.net
- Subject: [mg92796] Solve vs. NSolve
- From: SigmundV <sigmundv at gmail.com>
- Date: Mon, 13 Oct 2008 06:21:07 -0400 (EDT)
Dear group members, Consider f[a_, b_, c_, k_, t_] := With[{\[Alpha] = a k, \[Beta] = b k}, (x - \[Alpha] Cos[t])^2/a^2 + (y - (\[Beta] Sin[t] + c) - c)^2/b^2 - 1 == 0]; df[a_, b_, c_, k_, t_] := D[f[a, b, c, k, t], t]; and execute {x, y} /. Simplify@PowerExpand@Simplify@Solve[{f[1, 2, 1/2, 4/5, t], df[1, 2, 1/2, 4/5, t]}, {x, y}] // Chop // N and {x, y} /. Simplify@PowerExpand@Simplify@NSolve[{f[1, 2, 1/2, 4/5, t], df[1, 2, 1/2, 4/5, t]}, {x, y}] // Chop Simplify@PowerExpand@Chop@Simplify[% /. Cos[2 t] -> (1 - 2 Sin[t]^2)] respectively. As you see, Solve and NSolve yield two different solutions, with the solution from Solve being the correct one, as can be verified by plugging in to the equations -- the solution from NSolve does not satisfy the second equation, but only the first. Can anyone explain this behaviour to me? Best wishes, Sigmund Vestergaard
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- Re: Solve vs. NSolve