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MathGroup Archive 1993

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Re: untitled question

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Re: untitled question
  • From: wmm at chem.wayne.edu (Martin McClain)
  • Date: Thu, 7 Oct 93 11:13:26 EDT

Dear Sergio-  Try this:
Eliminate[{ (x-a)^2+(y-b)^2==r^2 , 
                    x^2+y^2==9 }, {y}] /. a_==b_ -> a-b==0
Then you can pull out the left side and put your own discriminant on it.
Regards- Martin McClain

>I am trying to use Eliminate to eliminate the unknown y in a system of 2 equations:
>
>In[1]:= Eliminate[{ (x-a)^2+(y-b)^2==r^2 , 
>                    x^2+y^2==9 },            {y}]
>
>           4    2           2      2
>Out[1]= r  + r  (-18 - 2 a  - 2 b  + 4 a x) == 
> 
>               2    4       2      2  2    4               3          2
>>    -81 - 18 a  - a  + 18 b  - 2 a  b  - b  + 36 a x + 4 a  x + 4 a b  x - 
> 
>         2  2      2  2
>>     4 a  x  - 4 b  x
>
>
>Is there any way to force Eliminate to produce a result in the form:
>
>	expr==0
>
>Or better yet there is any smart way to compute directly the discriminant of the second degree equation 
>(in x) Out[1]?
>
>Thank you.
>
>Sergio rescia






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