Circles

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

  1. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    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
    20,765
    2,536
    Have you tried graphing it?
     
  3. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Nope, I'll try that now.
     
  4. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
    834
    417
    Just graphed it - but I'm unsure what I'm supposed to do from here.

    Sorry about that.
     
  5. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    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
    513
    Should your equation not read?

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

    That is why have -4x + 12x?
     
    Sparky49 likes this.
  7. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
    834
    417
    Good spot - I'll change it now!
     
  8. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    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 Active Member

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

    (CP^{2}=) (5-2)^{2}+(k+6)^{2}<br />
 <br />
= k^{2}+12k+45

    Is that correct?
     
  10. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    Yes, it's correct. Sorry for the late reply. Did you manage to solve the rest?
     
  11. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    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!
     
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