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confused! AuBuC

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confused! AuBuC [#permalink]

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New post 07 Jun 2009, 09:24
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

I have been seeing two formulaes for AuBuC

1)AuBuC=A+B+C-(AintB+BintC+CintA)+AintBintC this makes perfect sense

2)AuBuC=A+B+C-(AintB+BintC+CintA)-2(AintBintC) ??? how is the second formula derived /when to use this

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iWillWin

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Re: confused! AuBuC [#permalink]

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New post 10 Jun 2009, 04:50
iwillwin wrote:
I have been seeing two formulaes for AuBuC

1)AuBuC=A+B+C-(AintB+BintC+CintA)+AintBintC this makes perfect sense

2)AuBuC=A+B+C-(AintB+BintC+CintA)-2(AintBintC) ??? how is the second formula derived /when to use this

Regards,
iWillWin


where did you find that second equation? It doesn't make any sense to me.

Even if it were correct, one will find it very difficult to put to use as the formula is not logical. Trust me, I've had a look at it for 30 mins now and nothing occurs to my mind as to where this formula 2 can be made use of. Seems wrong to me :shock:

Check yourself testing out formula 2 on a Venn diagram. \(A+B+C-(A \cap B+B \cap C+C \cap A)-(A \cap B \cap C)\) can't be shown on a Venn diagram forget about \(-2(A \cap B \cap C)\). So, I don't think you should be worrying about where to use it or if it is right or wrong or how it was derived
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Re: confused! AuBuC [#permalink]

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New post 10 Jun 2009, 14:33
This is where I found the second formula as well. They have an example too.

http://www.gmathacks.com/gmat-math/thre ... -sets.html

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Re: confused! AuBuC [#permalink]

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New post 11 Jun 2009, 05:40
nookway wrote:
This is where I found the second formula as well. They have an example too.

http://www.gmathacks.com/gmat-math/thre ... -sets.html


Thanks for the link, Nookway but this formula is tailored to a particular type of problem.

The formula written there holds no relevance to Venn Diagram representation. Venn Diagram way is different and this formula way is completely different! It is just a formula and note that 2*(all three) is very specific to a case where one has to deal with three sets.

For a problem with n sets, the formula would turn into Total = Group1 + Group2 + Group3 + ... + Groupn - (sum of {n-1} group overlaps) - {n-1}*(all n) + Neither

If you are comfortable with Venn Diagrams, you need not look into this formula at all 8-)

On a personal note, using the formula can get very messy. I tried and found that it leads to a lot of confusion resulting in waste of time. I suggest stick to your basics (Venn Diagram or Set representation). But that's just me. :)
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Re: confused! AuBuC   [#permalink] 11 Jun 2009, 05:40
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confused! AuBuC

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