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Try to use the circle formula!

For what value of c will the circle with equation \(x^2 + 6x + y^2 - 4y + c = 0\) have a radius of length 4?

 Aug 25, 2018
 #1
avatar+100587 
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x^2 + 6x + y^2 - 4y   =  -c     complete the square on x  and  y

 

x^2 - 6x  + 9  + y^2 - 4y + 4  =  -c + 9 + 4

 

Note that if the radius is 4.....then the right side must  =  r^2  = 4^2  = 16

 

So

 

-c + 9 + 4   =16

-c + 13   =16

-c  = 3

c  = -3

 

 

cool cool cool

 Aug 25, 2018

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