Re: Trig Solve

• To: mathgroup at smc.vnet.net
• Subject: [mg72420] Re: Trig Solve
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• 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

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 mimuw.edu.pl> wrote in message
> news:<emo7n0\$ffp\$1 at smc.vnet.net>...
>>
>> 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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