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Homework Help: Help with a question about collinear points

  1. Sep 7, 2008 #1
    Let P1, P2, ..., P5 be five points, no three of which are collinear. How many lines contain two of these five points?

    How I thought about solving this was by way of combinations.

    C5,2= (5*4*3*2)/2 = 60.

    So there would be 60 lines that contain 2 of the 5 points. Is this a good way to approach this problem or is there a better way?

    Thank you for your help!
    Last edited: Sep 7, 2008
  2. jcsd
  3. Sep 7, 2008 #2


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    Welcome to PF!

    Hi mathstudent88! Welcome to PF! :smile:

    (hmm … 60 is a very large number … :rolleyes:)
    No … combinations is exactly the right way! :smile:

    but … C5,2 = 5!/2!(5-2)! = … ? :wink:
  4. Sep 7, 2008 #3
    haha I forgot all about the (5-2)! part... thank you for pointing it out for me!!

    the answer is 10.

    Thanks again!!
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