Probability 4 Question 22

22. Three players, A,B and C, toss a coin cyclically in that order (i.e. A,B,C,A,B,C,A,B, ) till a head shows. Let p be the probability that the coin shows a head. Let α,β and γ be, respectively, the probabilities that A,B and C gets the first head. Prove that β=(1p)α. Determine α,β and γ (in terms of p ).

(1998,8 M)

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Answer:

Correct Answer: 22. 2330

Solution:

  1. Let q=1p= probability of getting the tail. We have,

α= probability of A getting the head on tossing firstly

=P(H1 or T1T2T3H4 or T1T2T3T4T5T6H7 or )

=P(H)+P(H)P(T)3+P(H)P(T)6+

=P(H)1P(T)3=p1q3

Also,

β= probability of B getting the head on tossing secondly

=P(T1H2 or T1T2T3T4H5 or T1T2T3T4T5T6T7H8 or )=P(H)[P(T)+P(H)P(T)4+P(H)P(T)7+]=P(T)[P(H)+P(H)P(T)3+P(H)P(T)6+]=qα=(1p)α=p(1p)1q3

Again, we have

α+β+γ=1γ=1(α+β)=1p+p(1p)1q3=1p+p(1p)1(1p)3=1(1p)3pp(1p)1(1p)3γ=1(1p)32p+p21(1p)3=p2p2+p31(1p)3

Also,

α=p1(1p)3,β=p(1p)1(1p)3



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