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

 Mar 23, 2021
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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

 

To have a radius  of 4....then

 

-c + 9 + 4  =    4^2

 

-c + 9 + 4 =  16

 

-c + 13  = 16

 

-c  =  16  -  13

 

-c  =  3

 

c  =  -3

 

 

cool cool cool

 Mar 23, 2021

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