# Need help finding (h,K,) and the radius of a circle

• October 20th 2012, 09:38 AM
M670
Need help finding (h,K,) and the radius of a circle
so my equation is x2+y2-4x+2y-4=0
i know the formula is (x-h)2+(y-k)2=r2

x2-4x+y2+2y-4=0

the Y is (y-2)2 which makes my k equal to positive 2 just having trouble with the h and radius
• October 20th 2012, 09:50 AM
skeeter
Re: Need help finding (h,K,) and the radius of a circle
$x^2+y^2-4x+2y-4 = 0$

$x^2 - 4x + y^2 + 2y = 4$

complete the square for x and y ...

$(x^2 - 4x + 4) + (y^2 + 2y + 1) = 4 + 4 + 1$

finish it ...
• October 20th 2012, 10:03 AM
Soroban
Re: Need help finding (h,K,) and the radius of a circle
Hello, M670!

Sorry, your algebra is off . . .

Quote:

$\text{Find the center and radius of the circle: }\:x^2+y^2-4x+2y-4\:=\:0$

We must "complete the square".

$\begin{array}{c}x^2 + y^2 - 4x + 2y - 4 \:=\:0 \\ \\ x^2 - 4x + y^2 + 2y \:=\:4 \\ \\ x^2 - 4x \,{\color{red}+\, 4} + y^2 + 2y {\color{blue}\,+\, 1} \:=\:4{\color{red}\,+\,4}{\color{blue}\,+\,1} \\ \\ (x-2)^2 +(y+1)^2 \:=\:9 \end{array}$

The circle has center $(2,\text{-}1)$ and radius $r = 3.$
• October 20th 2012, 10:33 AM
M670
Re: Need help finding (h,K,) and the radius of a circle
ok I understand that question but given this example here 5 x^2 - 10 x +5 y^2 - 8 y - 6 = 0 .

I worked it down to (x-1)^2 and (y-0.8)^2 =
I divided out 5 and was left with x^2-2x+y^2-1.6y-1.2=0
then its x^2-2x+1 + y^2-1.6y ?= 1.2-1 (?)
• October 20th 2012, 11:01 AM
Plato
Re: Need help finding (h,K,) and the radius of a circle
Quote:

Originally Posted by M670
ok I understand that question but given this example here 5 x^2 - 10 x +5 y^2 - 8 y - 6 = 0 .

I worked it down to (x-1)^2 and (y-0.8)^2 =
I divided out 5 and was left with x^2-2x+y^2-1.6y-1.2=0
then its x^2-2x+1 + y^2-1.6y ?= 1.2-1 (?)

Why do you have $1.2-1 (?)$?
It clearly should be $1.2+(1)+(.64)$
• October 20th 2012, 11:13 AM
M670
Re: Need help finding (h,K,) and the radius of a circle
Quote:

Originally Posted by Plato
Why do you have $1.2-1 (?)$?
It clearly should be $1.2+(1)+(.64)$

The ? was because I was unsure if it would be .64 or something esle
• October 20th 2012, 11:15 AM
M670
Re: Need help finding (h,K,) and the radius of a circle
How do you figure it out when you have a fraction or decimal to complete the squares?
• October 20th 2012, 11:23 AM
Plato
Re: Need help finding (h,K,) and the radius of a circle
Quote:

Originally Posted by M670
How do you figure it out when you have a fraction or decimal to complete the squares?

It is one-half the coefficient of the 'middle term' squared.
$\left(\frac{-1.6}{2}\right)^2=0.64$.