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For what value of c will the circle with equation x^2 + 8x + y^2 + 6y + c = 0 have a radius of length 3?

 Dec 17, 2020
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Complete  the square on x , y

 

(x^2 + 8x + 16) + (y^2 + 6y + 9)  =  -c + 16  + 9

 

(x + 4)^2   +  ( y  + 3)^2   =  25  - c

 

We need  25  - c =  3^2

 

So

 

25 - c     = 9

 

25  -  9  =    c

 

16  =   c

 

 

cool cool cool

 Dec 17, 2020

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