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Probability ( simple q )

  1. Jul 14, 2004 #1
    what does this mean?

    P (A or B but not both ( A and B ) ) ?

    does it mean
    P (A U B ) - P (A intersect B )?
  2. jcsd
  3. Jul 14, 2004 #2
    Yes that is correct. By the way, do you know how to draw "Venn's diagram"? That would make things a bit easier.
  4. Jul 14, 2004 #3
    i ask that question because i see my friend work
    she said it means

    P ( A U B ) - 2 P ( A intersect B )

    and she get the answer.
  5. Jul 14, 2004 #4
    Ok, I think it means P(A)+P(B)-2P(A intersects B).
  6. Jul 14, 2004 #5
    thats it. thanks.
  7. Jul 14, 2004 #6
    I believe your problem is essentially solved but it is helpful to construct Venn diagrams in such situations (as Wong suggested) to make some useful generalizations. Set theoretically, A or B but not not both A and B means that the region in the venn diagram that is common to both A and B is excluded (of course, the region described by the sample space - (A+B) is also excluded). This translates (in the "only" notation) to,

    (A only) \bigcup (B only)

    which really is

    (A \bigcap B') \bigcup (A' \bigcap B)
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