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Re: Help with Solve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112388] Re: Help with Solve
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Sun, 12 Sep 2010 03:12:03 -0400 (EDT)

On 11 Sep 2010, at 11:46, Eduardo Cavazos wrote:

> On Sep 9, 3:23 am, Eduardo  Cavazos <wayo.cava... at gmail.com> wrote:
>
>> Here's a couple of equations:
>>
>> eq1 == n*Sin[x] ==== (m*v^2)/r;
>> eq2 == n*Cos[x] ==== m*g;
>>
>> The goal is to solve for 'x'.
>>
>> I can do this in a roundabout way via:
>>
>>    Solve[eq1 /. Solve[eq2, n], x]
>
> I should point out that the answer that I get from using Solve this
> way is:
>
>    {{x -> ArcTan[v^2/(g r)]}}
>
> which is form the answer I'm looking for. :-) Again, I'm just
> wondering if Mathematica can do the work for me, instead of having to
> manually eliminate 'n' in a separate step.
>
> Sjoerd and Bob recommended using Reduce. Thanks for the tip! I tried
> to use Reduce but the results it produced were not in the above form.
>
> Ed
>

You can tell Solve to eliminate n before solving for x:

Solve[{eq2, eq1}, x, n]

(The answers are expressed in terms of ArcCos instead of ArcTan but are essentially equivalent to the one you got. Of course using Solve, unlike Reduce, not give you a complete solution)

Andrzej Kozlowski


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