equation of a circle

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czagara

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Sep 24, 2009
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Write the equation of the circle in standard form. Then sketch the circle.

x^2 + y^2 ? 2x + 2y ? 79 = 0

(x^2-2x+1) I am not really sure if i am doing this right and i don't really know what do to next.
 
Do you know how to complete the square in both x and y ?

That will lead to the form: (x - h)^2 + (y - k)^2 = r^2.
 
The standard equation for a circle (and someone can correct my on my use of constants) is
\(\displaystyle (x-h)^2 +(y-k)^2 = r^2\)

So you need to get it in this form by completing the square.

I'll give you a hint on how to start....

\(\displaystyle x^2 +y^2 -2x +2y -79 = 0\)

\(\displaystyle (x^2 -2x +1) + (y^2 +2y +1) -81 = 0\)

These two are equivalent, but the second is in an easier-to-see form for completing the square. Try this one and see if you can figure it out from here!

Hope this helps

Edited to switch the k and h to what mmm4444bot had :wink:
 
i have no idea what that means that you just wrote. I am very unfamiliar with how to do circles on a graph. I never learned anything about circles before. All i know how to do is the first step which i wrote x^2 - 2x +1
 
In the equation I added a total of two +1's , correct? therefore 1+1-81 = -79, which is what was there originally.
 
Re:

mmm4444bot said:
czagara said:
I never learned anything about circles before.
Click to expand...

That is one crappy course.
Click to expand...


All you need to know about circles is this equation:

\(\displaystyle (x-h)^2 + (y-k)^2 = r^2\)

Circles are defined by three things on a graph... the x-coordinate, y-coordinate and its radius which are represented by h, k and r, respectively
 
and yes this is the worst course i have ever taken, I am a 4.0 nursing student and this stuff is unbelieveable that they want us to do on a computer. Draw circles and parabolas, i have never even heard of this stuff!!!
 
czagara said:
does it go something like this?

(x-1)^2 + (y+1)^2=83???
Click to expand...


You are almost right-- where did you get 83 from? Remember from my original post? Look at that equation again, there is only a 81 there.
 
czagara said:
and yes this is the worst course i have ever taken, I am a 4.0 nursing student and this stuff is unbelieveable that they want us to do on a computer. Draw circles and parabolas, i have never even heard of this stuff!!!
Click to expand...


Eek! You've never heard of a circle? :? :? :?
 
Re: Re:

JuicyBurger said:
All you need to know about circles is this equation:

\(\displaystyle (x-h)^2 + (y-k)^2 = r^2\)

Circles are defined by three things on a graph... the x-coordinate, y-coordinate and its radius which are represented by h, k and r, respectively
Click to expand...
 
i meant a circle on a graph, no i have never heard of drawing circles on a graph before, but yes i have heard of a circle :roll:
 
Okay well now that you have made the equation contain an 81, you are entirely correct.

\(\displaystyle (x-1)^2 +(y+1)^2 = 81\)
 
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