(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A fair coin is tossed repeatedly and X is the number of tosses before the first head appears. You independently repeat the experiment, and Y is the number of tosses before the first head appears in the second sequence of tosses.

a. Give the probability mass function of X

b. Find P(X>n), for n≥1

c. Find P(X=Y)

d. Find P(X>Y)

2. Relevant equations

Geometric pmf: p(x) = p(1-p)^{x-1}

3. The attempt at a solution

a. I believe this should be P(X=x) = (1/2)^{x+1}. Is this correct so far?

b. P(X>n) = 1 - P(X≤n) = 1 - [itex]\sum \limits_{k=1}^n[/itex](1/2)^{x+1}

I feel like a closed form for this is expected. I know there's some kind of formula for sums of powers, but we never looked at it in class. So, I'm wondering if there's another way to do this question.

c. P(X=Y) = [itex]\sum \limits_{k=1}^∞[/itex](1/2)^{2x+2}

Is this correct? If so, same issue with explicitly evaluating the sum as with b.

d. Since the problem is symmetric with respect to X and Y, presumably the answer is:

P(X>Y) = 0.5*[1-P(X=Y)]

Thus, I need the closed form solution from c. This solution relies on the fact that X and Y have the same distribution. Is there a more general method for this?

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# Homework Help: Tossing a fair coin

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