# Math Is Fun Forum

Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫  π  -¹ ² ³ °

You are not logged in.

## #1 2006-11-14 02:55:15

unique
Member
Registered: 2006-10-04
Posts: 419

find the radius and coordinates of the center of the circle whose equation is

9x^2 + 9y^2 - 18x + 36y - 36 = 0

ok how do i do this...

Desi
Raat Key Rani !

Offline

## #2 2006-11-14 04:35:04

mathsyperson
Moderator
Registered: 2005-06-22
Posts: 4,900

Divide everything by 9 and group the x and y terms: x² - 2x + y² + 4y - 4 = 0

Complete the square for x and y: [(x-1)² - 1] + [(y+2)² - 4] - 4 = 0

Combine the constants: (x-1)² + (y+2)² = 3²

And there you have your standard circle form: (x-a)² + (y-b)² = r².

Can you work out the radius and centre coordinates now?

Why did the vector cross the road?
It wanted to be normal.

Offline

## #3 2006-11-14 09:35:26

unique
Member
Registered: 2006-10-04
Posts: 419

ok i know what you are saying...
you have given me the standard circle form which i can do
which i see in the book.
but in teh book we have numbers instead of letters..can i change teh letter into number and letters back again for me to do this?

liek in the book
the standard form is > (x + 3)^2 + (y - 3)^2 = 16
then they did
x^2 + 6x + 9 + y^2 - 6y + 9 = 16
and finished by collecting like terms
x^2 + y^2 + 6x - 6y + 2 = 0

so in our problem:
(x - a)^2 + (y - b)^2 = r^2 right
so we can do teh same way like the way they did in the book?
like
x^2 - a^2 + y^2 - b^2 + ay - ab = r^2 then what i think i have the wrong way in doing this...help

Desi
Raat Key Rani !

Offline

## #4 2006-11-15 04:39:37

unique
Member
Registered: 2006-10-04
Posts: 419

hello help anyone?

Desi
Raat Key Rani !

Offline

## #5 2016-12-15 20:41:37

Monox D. I-Fly
Member
From: Indonesia
Registered: 2015-12-02
Posts: 2,000

unique wrote:

so in our problem:
(x - a)^2 + (y - b)^2 = r^2 right
so we can do teh same way like the way they did in the book?
like
x^2 - a^2 + y^2 - b^2 + ay - ab = r^2 then what i think i have the wrong way in doing this...help

Where did you get that ay and -ab from? (x - a)^2 is x^2 - 2ab + a^2.

Actually I never watch Star Wars and not interested in it anyway, but I choose a Yoda card as my avatar in honor of our great friend bobbym who has passed away.
May his adventurous soul rest in peace at heaven.

Offline

## #6 2016-12-16 20:55:01

Bob
Registered: 2010-06-20
Posts: 9,469

hi Monox D. I-Fly

Looks to me like that poster just made a mistake clearing the brackets.  Should be:

There's a great page on this here:

http://www.mathsisfun.com/algebra/circle-equations.html

Bob

ps.  unique hasn't posted since 27th Feb 2007 so it's unlikely you'll get a response from the poster.  People join, post for a while, and then stop.  If you find an old post like this one, it's worth clicking the poster's name and then their posts to see when the last one was.  That way you'll know if they are continuing to be an active member.

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Offline

## #7 2016-12-17 12:53:38

Monox D. I-Fly
Member
From: Indonesia
Registered: 2015-12-02
Posts: 2,000

bob bundy wrote:

hi Monox D. I-Fly

Looks to me like that poster just made a mistake clearing the brackets.  Should be:

There's a great page on this here:

http://www.mathsisfun.com/algebra/circle-equations.html

Bob

Well, since no one has responded in 10 years...

bob bundy wrote:

ps.  unique hasn't posted since 27th Feb 2007 so it's unlikely you'll get a response from the poster.  People join, post for a while, and then stop.  If you find an old post like this one, it's worth clicking the poster's name and then their posts to see when the last one was.  That way you'll know if they are continuing to be an active member.

Even though he isn't here anymore, we can still help others who read this post and want answers, no?

Actually I never watch Star Wars and not interested in it anyway, but I choose a Yoda card as my avatar in honor of our great friend bobbym who has passed away.
May his adventurous soul rest in peace at heaven.

Offline