Circles

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

  1. Sparky49

    Thread Starter Well-Known Member

    Jul 16, 2011
    836
    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
    20,993
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    Have you tried graphing it?
     
  3. Sparky49

    Thread Starter Well-Known Member

    Jul 16, 2011
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    417
    Nope, I'll try that now.
     
  4. Sparky49

    Thread Starter Well-Known Member

    Jul 16, 2011
    836
    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
    1,333
    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
    519
    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 Well-Known Member

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

    Thread Starter Well-Known Member

    Jul 16, 2011
    836
    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
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    417
    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
    5,151
    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
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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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