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Math Help - Need a little freshened up

  1. #1
    Newbie
    Joined
    Nov 2008
    Posts
    14

    Need a little freshened up

    In a question like: C(n,2)=45

    n(n-1)=90
    n2-n-90=0
    (n-10)(n+9)=0 <- how do you know which to make the positive and which to make the negative?
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  2. #2
    Member
    Joined
    Oct 2008
    Posts
    124
    solve n+9=0 and n-10=0. n=10. 10 is positive. Just solve both after setting both to 0.
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