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Set Theory!

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Set Theory! [#permalink] New post 07 Jun 2011, 07:48
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I have a clearifying question about set theory formulas, if you all don't mind.

1. For 3 sets A, B, and C: P(AuBuC) = P(A) + P(B) + P(C) – P(AnB) – P(AnC) – P(BnC) + P(AnBnC)

2. No of persons in atleast one set = P(A) + P(B) + P(C) - P(AnB) - P(AnC) - P(BnC) + 2 P(AnBnC)

Why these two formulas are different? Isn't "at least in one set" equal to union of all (AuBuC)?

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Re: Set Theory! [#permalink] New post 07 Jun 2011, 08:06
bellcurve wrote:
I have a clearifying question about set theory formulas, if you all don't mind.

1. For 3 sets A, B, and C: P(AuBuC) = P(A) + P(B) + P(C) – P(AnB) – P(AnC) – P(BnC) + P(AnBnC)

2. No of persons in atleast one set = P(A) + P(B) + P(C) - P(AnB) - P(AnC) - P(BnC) + 2 P(AnBnC)

Why these two formulas are different? Isn't "at least in one set" equal to union of all (AuBuC)?

Thanks,
BC


I think first one is good for at least one person as well.

No idea where can we use the second formula.
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Re: Set Theory! [#permalink] New post 07 Jun 2011, 09:29
Does anybody think differently?
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Re: Set Theory! [#permalink] New post 07 Jun 2011, 09:47
P(A u B u C)= P(A) + P(B) + P(C) – P(A n B) – P(A n C) – P(B n C) + 2 P(A n B n C) - atleast one
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Re: Set Theory! [#permalink] New post 07 Jun 2011, 11:32
sudhir18n, can you please clarify?

If I solve your equation using the equations above, I would get

P(A u B u C)= P(A n B n C)

Which is definatley not true. What am I missing here?
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Re: Set Theory! [#permalink] New post 07 Jun 2011, 11:46
bellcurve wrote:
sudhir18n, can you please clarify?

If I solve your equation using the equations above, I would get

P(A u B u C)= P(A n B n C)

Which is definatley not true. What am I missing here?


use this
http://grockit.com/blog/gmat/2011/01/28 ... et-theory/
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Re: Set Theory! [#permalink] New post 07 Jun 2011, 12:38
The link says the same thing. I have the formulas, its just that I am not sure why formula for P(AuBuC) is different than "at least one." Aren't those refering to the same things? It seems like, "at least one" is obtained by adding P(AnBnC) to P(AuBuC). What can be included in the "at least one" that can not be included in P(AuBuC)?
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Re: Set Theory! [#permalink] New post 10 Jun 2011, 09:01
Can anybody shed some light?
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Re: Set Theory! [#permalink] New post 13 Jun 2011, 19:55
bellcurve wrote:
Anybody?


I think that 'at least one' should be the same as union. But they seem to have something else in mind (I don't have the faintest clue what that is). In terms of the diagram, the area that corresponds to 'at least one set' according to them, doesn't represent anything.
You may want to ask them for clarification on their blog post.
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Re: Set Theory!   [#permalink] 13 Jun 2011, 19:55
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