# Circles

Discussion in 'Math' started by Sparky49, Jan 2, 2012.

1. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
835
417
Hi everyone,

now I know this is easy peasy for you guys - but bear with me!

I'll write out the whole question first.

Phew!

I completely understand parts a and b, but c has confused me a little. I've tried thinking about it (alot!) but I am just unsure as to what approach I need for c. It's only worth a couple of marks, so it should be fairly easy - but I'm not seeing what I need to do!

Could you point me in the right direction?

Thanks,

Sparky

Last edited: Jan 2, 2012
2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Have you tried graphing it?

3. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
835
417
Nope, I'll try that now.

4. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
835
417
Just graphed it - but I'm unsure what I'm supposed to do from here.

5. ### DerStrom8 Well-Known Member

Feb 20, 2011
2,428
1,329
After you graph it, you should get a better picture of what it looks like. From there, it's mainly just plug and play. Insert the values (k is the y value of the center coordinate; center is (h,k)) in the formula and show that it gives the correct result. Then, find where the circle intersects the Y axis (find possible values of 'k').

I hope this helps. Good luck!
Regards,
Der Strom

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515

${x^2} + {y^2} - 4x + 12y + 15 = 0$

That is why have -4x + 12x?

Sparky49 likes this.
7. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
835
417
Good spot - I'll change it now!

8. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
835
417
Okay, I think I'm understanding this more - but what values are P and C?

They are points - not numbers?

Sorry if I'm being a bit - slow.

9. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
835
417
I see how to do it now!

$(CP^{2}=) (5-2)^{2}+(k+6)^{2}

= k^{2}+12k+45$

Is that correct?

10. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Yes, it's correct. Sorry for the late reply. Did you manage to solve the rest?

11. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
835
417
No problem!

I got the rest pretty quickly - it was mainly the twisted notation which confused me. It suddenly clicked that CP'2' was the radius squared!