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Probability question

2007 · Shift 1 · Q32
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  5. /2007 · Shift 1 · Q32

Probability question

2007 · Shift 1 · Q32

JEE AdvancedMathematicsProbabilityMCQ+3 / −1
Let H 1_11​, H 2_22​, ..., H n_nn​ be mutually exclusive and exhaustive events with P(H i_ii​) > 0, i = 1, 2, ..., n. Let E be any other event with 0 Statement 1 : P(H i_ii​ | E) > P(E | H i_ii​). P(H i_ii​) for i=1,2,...,ni=1,2,...,ni=1,2,...,n. Statement 2 : ∑i=1nP(Hi)=1\sum\limits_{i = 1}^n {P({H_i}) = 1}i=1∑n​P(Hi​)=1.
  1. A
    Statement 1 is True, Statement 2 is True, Statement 2 is a CORRECT explanation for Statement 1
  2. B
    Statement 1 is True, Statement 2 is True, Statement 2 is NOT a CORRECT explanation for Statement 1
  3. C
    Statement 1 is True, Statement 2 is False
  4. D
    Statement 1 is False, Statement 2 is True
View written solutionFree

Correct answer: D

Analysis of Statement 2

  1. Definition of Mutually Exclusive and Exhaustive Events:

    • The events H1,H2,...,HnH_1, H_2, ..., H_nH1​,H2​,...,Hn​ are given to be mutually exclusive, which means Hi∩Hj=∅H_i \cap H_j = \emptysetHi​∩Hj​=∅ for any i≠ji \neq ji=j.
    • They are also exhaustive, which means their union covers the entire sample space, i.e., H1∪H2∪...∪Hn=SH_1 \cup H_2 \cup ... \cup H_n = SH1​∪H2​∪...∪Hn​=S.
  2. Probability of the Sample Space:

    • By the axioms of probability, the probability of the entire sample space is 1, i.e., P(S)=1P(S) = 1P(S)=1.
  3. Sum of Probabilities:

    • Since the events are exhaustive, P(H1∪H2∪...∪Hn)=P(S)=1P(H_1 \cup H_2 \cup ... \cup H_n) = P(S) = 1P(H1​∪H2​∪...∪Hn​)=P(S)=1.
    • Since the events are mutually exclusive, the probability of their union is the sum of their individual probabilities: P(H1∪H2∪...∪Hn)=∑i=1nP(Hi)P(H_1 \cup H_2 \cup ... \cup H_n) = \sum_{i=1}^{n} P(H_i)P(H1​∪H2​∪...∪Hn​)=∑i=1n​P(Hi​)
    • Combining these facts, we get: ∑i=1nP(Hi)=1\sum_{i=1}^{n} P(H_i) = 1∑i=1n​P(Hi​)=1
  4. Conclusion for Statement 2:

    • Statement 2 is a direct consequence of the definition of mutually exclusive and exhaustive events. Therefore, Statement 2 is True.

Analysis of Statement 1

  1. The Inequality:

    • Statement 1 claims that P(Hi∣E)>P(E∣Hi)⋅P(Hi)P(H_i | E) > P(E | H_i) \cdot P(H_i)P(Hi​∣E)>P(E∣Hi​)⋅P(Hi​) for all i=1,2,...,ni=1, 2, ..., ni=1,2,...,n.
  2. Applying Bayes' Theorem:

    • Bayes' theorem states that P(Hi∣E)=P(E∣Hi)P(Hi)P(E)P(H_i | E) = \frac{P(E | H_i) P(H_i)}{P(E)}P(Hi​∣E)=P(E)P(E∣Hi​)P(Hi​)​.
    • Substituting this into the inequality from Statement 1: P(E∣Hi)P(Hi)P(E)>P(E∣Hi)⋅P(Hi)\frac{P(E | H_i) P(H_i)}{P(E)} > P(E | H_i) \cdot P(H_i)P(E)P(E∣Hi​)P(Hi​)​>P(E∣Hi​)⋅P(Hi​)
  3. Analyzing the Inequality:

    • Let's analyze this inequality. We are given P(Hi)>0P(H_i) > 0P(Hi​)>0.

    • Case 1: P(E∣Hi)>0P(E | H_i) > 0P(E∣Hi​)>0 In this case, the term P(E∣Hi)P(Hi)P(E | H_i) P(H_i)P(E∣Hi​)P(Hi​) is positive. We can divide both sides of the inequality by this term without changing the direction of the inequality: 1P(E)>1\frac{1}{P(E)} > 1P(E)1​>1 This is equivalent to 1>P(E)1 > P(E)1>P(E), or P(E)<1P(E) < 1P(E)<1. The problem states that 0<P(E)<10 < P(E) < 10<P(E)<1, so this condition is met. Thus, if P(E∣Hi)>0P(E | H_i) > 0P(E∣Hi​)>0, Statement 1 is true.

    • Case 2: P(E∣Hi)=0P(E | H_i) = 0P(E∣Hi​)=0 This occurs if the event EEE and the event HiH_iHi​ are mutually exclusive, i.e., E∩Hi=∅E \cap H_i = \emptysetE∩Hi​=∅. Since EEE is described as 'any other event', this is a possibility. If P(E∣Hi)=0P(E | H_i) = 0P(E∣Hi​)=0, the right side of the inequality in Statement 1 is 0⋅P(Hi)=00 \cdot P(H_i) = 00⋅P(Hi​)=0. The left side is P(Hi∣E)=P(Hi∩E)P(E)=P(E∩Hi)P(E)=0P(E)=0P(H_i | E) = \frac{P(H_i \cap E)}{P(E)} = \frac{P(E \cap H_i)}{P(E)} = \frac{0}{P(E)} = 0P(Hi​∣E)=P(E)P(Hi​∩E)​=P(E)P(E∩Hi​)​=P(E)0​=0 (since P(E)>0P(E)>0P(E)>0). The inequality becomes 0>00 > 00>0, which is false.

  4. Counterexample:

    • Since Statement 1 must hold for all i=1,2,...,ni=1, 2, ..., ni=1,2,...,n, we can show it is false by finding a single case where it fails.
    • Let the sample space be S={1,2,3,4}S = \{1, 2, 3, 4\}S={1,2,3,4} with a uniform probability distribution, P({k})=1/4P(\{k\}) = 1/4P({k})=1/4 for each outcome kkk.
    • Let H1={1,2}H_1 = \{1, 2\}H1​={1,2} and H2={3,4}H_2 = \{3, 4\}H2​={3,4}. These are mutually exclusive and exhaustive. P(H1)=1/2>0P(H_1) = 1/2 > 0P(H1​)=1/2>0 and P(H2)=1/2>0P(H_2) = 1/2 > 0P(H2​)=1/2>0.
    • Let the event E={1,2}E = \{1, 2\}E={1,2}. Then P(E)=1/2P(E) = 1/2P(E)=1/2, satisfying 0<P(E)<10 < P(E) < 10<P(E)<1.
    • Now let's check the inequality for i=2i=2i=2.
      • E∩H2={1,2}∩{3,4}=∅E \cap H_2 = \{1, 2\} \cap \{3, 4\} = \emptysetE∩H2​={1,2}∩{3,4}=∅. Therefore, P(E∩H2)=0P(E \cap H_2) = 0P(E∩H2​)=0.
      • Right side of the inequality: P(E∣H2)⋅P(H2)=P(E∩H2)P(H2)⋅P(H2)=P(E∩H2)=0P(E | H_2) \cdot P(H_2) = \frac{P(E \cap H_2)}{P(H_2)} \cdot P(H_2) = P(E \cap H_2) = 0P(E∣H2​)⋅P(H2​)=P(H2​)P(E∩H2​)​⋅P(H2​)=P(E∩H2​)=0.
      • Left side of the inequality: P(H2∣E)=P(H2∩E)P(E)=01/2=0P(H_2 | E) = \frac{P(H_2 \cap E)}{P(E)} = \frac{0}{1/2} = 0P(H2​∣E)=P(E)P(H2​∩E)​=1/20​=0.
      • The inequality for i=2i=2i=2 becomes 0>00 > 00>0, which is false.
  5. Conclusion for Statement 1:

    • Since we have found a valid scenario where the inequality does not hold for one of the events HiH_iHi​, Statement 1 is False.

Final Conclusion

  • Statement 1 is False.
  • Statement 2 is True.

This corresponds to option D.

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