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Homework Help: Marginal probability mass function

  1. Oct 12, 2012 #1
    A biased coin lands heads with probability p and tails with probability q. An experiment
    consists of tossing the coin until a second head appears. Let T1 denote the number of
    tosses until the first head appears and let T2 denote the number of tosses (counted from
    the start) until the second head appears. For example, in the outcome TTHTTTH we
    would have T1 = 3 and T2 = 7. Find the marginal PMF for each of T1 and T2.

    I know the joint mass function is
    P(T1 = j, T2 = n) = p2qn-2 for 1 < j < n -1

    I need assistance in finding the marginal PMF from there...
  2. jcsd
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