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Homework Help: Combinatorics problem

  1. Oct 17, 2004 #1
    :zzz: :zzz: :zzz:
    Last edited: Oct 18, 2004
  2. jcsd
  3. Oct 17, 2004 #2
    2. Reduce the problem to a simpler one and find a pattern. For example, in a 2x2 square only 4 triangles have this property.
    3. Could you give the exact wording of the question or rephrase it? I am unsure if you mean one quarter of all subsets of integers 1,2,3...m with three elements or not.
  4. Oct 17, 2004 #3
    1> How many arrangements are there such that no two A's are beside each other?
    2> shouldn't be a problem .... straightforward (only thing to remember ... 3 points cannot be collinear)
    3> how many 3-subsets can you form? (call this A)
    How many 3-subsets can contain integer 5? (call this B)
    1/4 * A = B ... solve for m
    4> no three segments cross at any point ....
    therefore atmost two segments can cross each other ...
    and every crossing of two segments produces 1 intersection point ...
    so ??

    -- AI
  5. Oct 17, 2004 #4
    a subset with 3 elements

    TR.. im so lost

    could u explain it a lil more?

    and number one i know its ez but I cant seem to get it
  6. Oct 17, 2004 #5
    a .. First arrange the B's and y's? in how many ways can u do this?
    b .. amongst 6 B's and 7 y's there are 14 places where i can place A (why do i do this ? u see this way no two A will be beside each other)
    c .. in how many ways can i arrange the A's in these 14 places

    Can u proceed from here .... ??

    Look at what i gave in the brackets ... can u see how to pick the points then?

    Can u find A and B? (atleast A is pretty simple)

    How many segments are given?
    In how many ways can i choose 2 segments out of the given number of segments?

    -- AI
  7. Oct 17, 2004 #6
    TR.. ur help is greatly appreciated

    you can arrange this in 13! ways because thats how many total letters there are?

    i LOST you from here :(

    This i dont get at all

    got this thanks

    and the last one i dont get at all too

    man... i raped the test on this unit, but the teahers chellenge sets are HARD
  8. Oct 17, 2004 #7
  9. Oct 18, 2004 #8
    can the mods delte this thread pleez
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