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# im new and need help :( Watch

1. hello, i am new to this..hope i am posting in the right section

any way here goes:

"The radius of the circle x^2 + y^2 + 10x -8y + c = 0 is 8.

Find c"

Any ideas?

Thanks
2. (Original post by lolman42)
hello, i am new to this..hope i am posting in the right section

any way here goes:

"The radius of the circle x^2 + y^2 + 10x -8y + c = 0 is 8.

Find c"

Any ideas?

Thanks
What is the general equation of a circle?
3. Arrange it into the form (x+a)^2 + (y+b)^2 = r - get this by completing the square. You should have a function of c as your radius, which you know is eight. Then solve algebraically.
4. (Original post by tory88)
Arrange it into the form (x+a)^2 + (y+b)^2 = r - get this by completing the square. You should have a function of c as your radius, which you know is eight. Then solve algebraically.
thanks can I ask you another question?
5. (Original post by tory88)
Arrange it into the form (x+a)^2 + (y+b)^2 = r - get this by completing the square. You should have a function of c as your radius, which you know is eight. Then solve algebraically.
I do this

(x+5)^2 + (y-4)^2 = 16 + 25 - c

then do I do 16+25-c=8^2 ?

thanks
6. (Original post by lolman42)
I do this

(x+5)^2 + (y-4)^2 = 16 + 25 - c

then do I do 16+25-c=8^2 ?

thanks
Yes.
7. (Original post by lolman42)
thanks can I ask you another question?
Sure, anything to put off my own revision XD

(Original post by lolman42)
I do this

(x+5)^2 + (y-4)^2 = 16 + 25 - c

then do I do 16+25-c=8^2 ?

thanks
Yep, that's right

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