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2009 Paper 2 Question 4c

R1PPU

Posted 20 May 2010 - 12:07 PM

Hi, i'm doing this question just now, and what i dont understand is how do you know to use (5,10) and (-19,-22) as the centre's of the other circles? From the way the question is worded, you can use any point as your centres.

Cheers for any help in advance.

Marcus

Posted 20 May 2010 - 01:21 PM

View PostR1PPU, on 20 May 2010 - 12:07 PM, said:

Hi, i'm doing this question just now, and what i dont understand is how do you know to use (5,10) and (-19,-22) as the centre's of the other circles? From the way the question is worded, you can use any point as your centres.

Cheers for any help in advance.

You can't use any point as the centre because then the C2 and C3 would cross C1 more than once, therefore the centres of C2 and C3 would have to lie on the line of the diametre PQ 20 units from Q, hence (5,10) and (-19,-22), if you don't believe me try drawing a few circles with centres elsewhere and you'll see that they have to cross multiple times (or don't go through Q).

Apparently you weren't the only one to be confused by this quesion
Under "Areas which candidates found demanding"

R1PPU

Posted 20 May 2010 - 01:24 PM

View PostMarcus, on 20 May 2010 - 01:21 PM, said:

View PostR1PPU, on 20 May 2010 - 12:07 PM, said:

Hi, i'm doing this question just now, and what i dont understand is how do you know to use (5,10) and (-19,-22) as the centre's of the other circles? From the way the question is worded, you can use any point as your centres.

Cheers for any help in advance.

You can't use any point as the centre because then the C2 and C3 would cross C1 more than once, therefore the centres of C2 and C3 would have to lie on the line of the diametre PQ 20 units from Q, hence (5,10) and (-19,-22), if you don't believe me try drawing a few circles with centres elsewhere and you'll see that they have to cross multiple times (or don't go through Q).

Apparently you weren't the only one to be confused by this quesion
Under "Areas which candidates found demanding"


Ah yeah, they would intersect then, not "touch". I get it now, thanks :)

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