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Question 31 (B) A random variable 𝑋 can take all non - negative integral values and the probability that 𝑋 takes the value π‘Ÿ is proportional to 5^(βˆ’π‘Ÿ). Find 𝑃(𝑋<3).It’s given that P(X = r) is proportional to 5^(βˆ’π‘Ÿ) i.e. P(X = r) ∝ 5^(βˆ’π‘Ÿ) So, we can write P(X = r) = k 5^(βˆ’π‘Ÿ) = π’Œ/πŸ“^𝒓 Where k is some constant Thus, 𝑷(𝒓=𝟎)=π‘˜/5^0 = π‘˜/1 = π’Œ 𝑷(𝒓=𝟏)=π‘˜/5^1 = π’Œ/πŸ“ 𝑷(𝒓=𝟐)=π‘˜/5^2 𝑷(𝒓=πŸ‘)=π‘˜/5^3 First term = a = 1 Common ratio = r = (1/5)/1=𝟏/πŸ“ Now, Sum of infinite GP = π‘Ž/(1 βˆ’ π‘Ÿ) = 1/(1 βˆ’ 1/5) = 1/(4/5) = πŸ“/πŸ’ Now, π’Œ(𝟏+𝟏/πŸ“+𝟏/πŸ“^𝟐 +𝟏/πŸ“^πŸ‘ +…)=𝟏 Putting values π‘˜ Γ—5/4=1 π’Œ=πŸ’/πŸ“ We need to find 𝑃(𝑋<3). Thus, 𝑷(𝑿<πŸ‘)=𝑃(0)+𝑃(1)+𝑃(2) =π’Œ+π’Œ/πŸ“+π’Œ/πŸ“^𝟐 =π‘˜(1+1/5+1/5^2 ) =π‘˜(1+1/5+1/25) =π‘˜((25 + 5 + 1)/25) =π‘˜ Γ—31/25 Putting π’Œ=πŸ’/πŸ“ =4/5 Γ—31/25 =πŸπŸπŸ’/πŸπŸπŸ“

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Davneet Singh

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 14 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.