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Re: Trig Solve

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  • Subject: [mg72420] Re: Trig Solve
  • From: Andrzej Kozlowski <akoz at>
  • Date: Wed, 27 Dec 2006 05:39:09 -0500 (EST)
  • References: <000001c7284e$3a11be70$6401a8c0@test>

Obviously on can eliminate x, but what one will get is just one of  
the pair of equations one has t start with:

Eliminate[{x, y} == {Cos[th]*(3 + Cos[th/2 + v]),
     (3 + Cos[th/2 + v])*Sin[th]}, {x}]

(Cos[th/2 + v] + 3)*Sin[th] == y

Solve won't do this as it checks that the equations are non-algebraic  
and refuses to have anything to do with them. One can deal with trig  
problems of this type by using TrigExpand and Groebner basis, but  
only when the problem makes sense and this one (as it stands)  does not.

Andrzej Kozlowski

On 26 Dec 2006, at 02:58, Steven Shippee wrote:

> So in the end, this person should only have an "x" or a "y", but  
> not both,
> correct?
> "Andrzej Kozlowski" <akoz at> wrote in message
> news:<emo7n0$ffp$1 at>...
>> On 24 Dec 2006, at 17:40, Narasimham wrote:
>>> Could not get solution for easier implicit trig. relation
>>> Solve[{x, y} == {Cos[th]*(3 + Cos[th/2 + v]), (3 + Cos[th/2 +
>>> v])*Sin[th]}, {th, v}, {x, y}]
>>> Thanks in advance for indication for solutions.
>>> Narasimham
>> It is completely unclear to me what you mean be a "solution" to this
>> equation. What are you trying to "solve' in terms of what?
>> Note that the code you have written is actually telling Solve to
>> eliminate x and y
>> ?Solve
>> Solve[eqns, vars] attempts to solve an equation or set of
>> equations for the variables vars. Solve[eqns, vars, elims] attempts
>> to solve
>> the equations for vars, eliminating the variables elims.
>> which is obviously impossible.
>> Andrzej Kozlowski

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