equation of a circle  TOPIC_SOLVED

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equation of a circle

Postby prealan01 on Fri Mar 02, 2012 1:41 am

Need help with finding the center radius form of the circle 9x^+9y^+18y-12y-131=0. I can work these out when I can factor out the same number but, this one is giving me trouble.
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Postby stapel_eliz on Fri Mar 02, 2012 11:19 am

prealan01 wrote:Need help with finding the center radius form of the circle 9x^+9y^+18y-12y-131=0. I can work these out when I can factor out the same number but, this one is giving me trouble.

There is a 9 multiplied on each of the two squared terms. What other numbers are you trying to factor out? :confused:
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Re: equation of a circle

Postby prealan01 on Fri Mar 02, 2012 9:13 pm

I made a typo, sorry. If I rearrange it to read 9x^+18x+9y^-12y=131 I know I need to factor out a number before completing the square on both sides, but if I factor out a 3 it's got me confused.
Example: 4x^+12x+4y^+24y-99=0 move the 99 and factor out a 4, then complete the square both sides
4(x^+3x+9/4)+4(y^+6y+9)=99+4(9/4)+4(9)
4(x+3/2)^+4(y+3)^=144 divide both sides by 4
(x-3/2)^+(y+3)^=36
(-3/2,-3) radius 6
The first example just isn't working out for me.
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Postby stapel_eliz on Sat Mar 03, 2012 12:15 am

You've got 9's in front of both squared terms. I don't know where you're getting a 3 from...? Instead, try factoring out the number in front of both squared terms, just like you have for every other exercise of this type. :wink:
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Re: equation of a circle

Postby prealan01 on Sat Mar 03, 2012 4:09 am

That's where I'm having a problem. You can't factor out 9 because of the -12y. This problem isn't working out just as the other one I gave as an example.
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  TOPIC_SOLVED

Postby stapel_eliz on Sat Mar 03, 2012 12:29 pm

prealan01 wrote:That's where I'm having a problem. You can't factor out 9 because of the -12y.

You can divide through by 9. You'll just end up with some fractions. :wink:
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