The series could continue for ever so you need the sum to infinity of a geometric series.

Here 'a' is the first term 4/5 and r is 1/5

http://www.mathsisfun.com/algebra/seque … etric.html

Bob

]]>*Flip a coin until heads show, assume that the probability of heads on one flip is 4/5. We define a random variable X = the number of flips.*

*a) What are the possible values of X?b) Find the probability distribution for X: give the first four values and then find a general formula for the probability that X = nc) Prove that the sum of all probabilities is 1 using the formula for the sum of a geometric series.*

My answers:

a) X=1,2,3,...,n

b)

p(1) = 4/5

p(2) = 4/25

p(3) = 4/125

p(n) = ((1/5)^n-1) * (4/5)