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Probability question

2008 · Shift 1 · Q28
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  5. /2008 · Shift 1 · Q28

Probability question

2008 · Shift 1 · Q28

JEE AdvancedMathematicsProbabilityMCQ+3 / −1
Consider the system of equations ax+by=0;cx+dy=0,ax+by=0; cx+dy=0,ax+by=0;cx+dy=0, where a,b,c,da,b,c,da,b,c,d ∈{0,1}\in \left\{ {0,1} \right\}∈{0,1} STATEMENT - 1 : The probability that the system of equations has a unique solution is 38.{3 \over 8}.83​. and STATEMENT - 2 : The probability that the system of equations has a solution is 1.1.1.
  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: B

Step-by-step Solution:

1. Analyze the System of Equations and Sample Space

The given system of equations is: ax+by=0ax + by = 0ax+by=0 cx+dy=0cx + dy = 0cx+dy=0 This is a system of two homogeneous linear equations in two variables, x and y. The coefficients a,b,c,da, b, c, da,b,c,d are chosen from the set {0,1}\{0, 1\}{0,1}. Since each of the four coefficients can take 2 possible values (0 or 1), the total number of possible systems of equations is 2×2×2×2=24=162 \times 2 \times 2 \times 2 = 2^4 = 162×2×2×2=24=16. This is the size of our sample space.

2. Evaluate STATEMENT - 1

STATEMENT - 1 says: "The probability that the system of equations has a unique solution is 3/83/83/8."

A system of homogeneous linear equations has a unique solution (the trivial solution x=0,y=0x=0, y=0x=0,y=0) if and only if the determinant of the coefficient matrix is non-zero. The coefficient matrix is A=(abcd)A = \begin{pmatrix} a & b \\ c & d \end{pmatrix}A=(ac​bd​). The condition for a unique solution is det⁡(A)=ad−bc≠0\det(A) = ad - bc \neq 0det(A)=ad−bc=0.

We need to find the number of combinations of (a,b,c,d)(a, b, c, d)(a,b,c,d) from {0,1}4\{0, 1\}^4{0,1}4 for which ad−bc≠0ad - bc \neq 0ad−bc=0. Since a,b,c,d∈{0,1}a, b, c, d \in \{0, 1\}a,b,c,d∈{0,1}, the products adadad and bcbcbc can only be 0 or 1.

For ad−bc≠0ad - bc \neq 0ad−bc=0, we have two possible cases:

  • Case 1: ad−bc=1ad - bc = 1ad−bc=1 This implies ad=1ad=1ad=1 and bc=0bc=0bc=0.

    • For ad=1ad=1ad=1, we must have a=1a=1a=1 and d=1d=1d=1. This is 1 possibility for the pair (a,d)(a,d)(a,d).
    • For bc=0bc=0bc=0, at least one of bbb or ccc must be 0. The possible pairs for (b,c)(b,c)(b,c) are (0,0),(0,1),(1,0)(0,0), (0,1), (1,0)(0,0),(0,1),(1,0). This gives 3 possibilities.
    • The total number of systems in this case is 1×3=31 \times 3 = 31×3=3.
  • Case 2: ad−bc=−1ad - bc = -1ad−bc=−1 This implies ad=0ad=0ad=0 and bc=1bc=1bc=1.

    • For bc=1bc=1bc=1, we must have b=1b=1b=1 and c=1c=1c=1. This is 1 possibility for the pair (b,c)(b,c)(b,c).
    • For ad=0ad=0ad=0, at least one of aaa or ddd must be 0. The possible pairs for (a,d)(a,d)(a,d) are (0,0),(0,1),(1,0)(0,0), (0,1), (1,0)(0,0),(0,1),(1,0). This gives 3 possibilities.
    • The total number of systems in this case is 3×1=33 \times 1 = 33×1=3.

The total number of favorable outcomes for a unique solution is the sum of outcomes from both cases: 3+3=63 + 3 = 63+3=6.

The probability of the system having a unique solution is: P(unique solution)=Favorable OutcomesTotal Outcomes=616=38P(\text{unique solution}) = \frac{\text{Favorable Outcomes}}{\text{Total Outcomes}} = \frac{6}{16} = \frac{3}{8}P(unique solution)=Total OutcomesFavorable Outcomes​=166​=83​ Therefore, STATEMENT - 1 is True.

3. Evaluate STATEMENT - 2

STATEMENT - 2 says: "The probability that the system of equations has a solution is 1."

The given system is a homogeneous system of linear equations. A homogeneous system always has at least one solution, which is the trivial solution (x,y)=(0,0)(x,y) = (0,0)(x,y)=(0,0). We can verify this by substituting x=0x=0x=0 and y=0y=0y=0 into the equations: a(0)+b(0)=0a(0) + b(0) = 0a(0)+b(0)=0 c(0)+d(0)=0c(0) + d(0) = 0c(0)+d(0)=0 These equations hold true for any values of a,b,c,da, b, c, da,b,c,d. Since the system always has a solution for all 16 possible combinations of coefficients, the event "the system has a solution" is a certain event. The probability of a certain event is 1. Therefore, STATEMENT - 2 is True.

4. Analyze the Relationship Between the Statements

We have determined that both STATEMENT - 1 and STATEMENT - 2 are true. Now, we must check if STATEMENT - 2 is the correct explanation for STATEMENT - 1.

  • STATEMENT - 1 calculates the probability of a unique solution. The value 3/83/83/8 is derived from counting the specific combinations of coefficients where the determinant ad−bcad-bcad−bc is non-zero.
  • STATEMENT - 2 states a general property of homogeneous systems: they are always consistent (i.e., they always have at least one solution). The system can have either a unique solution or infinitely many solutions, but it never has no solution.

The fact that the system always has a solution (STATEMENT - 2) is a necessary precondition, but it does not explain why the probability of the solution being unique is specifically 3/83/83/8. The reason for this probability is the combinatorial result from step 2. Therefore, STATEMENT - 2 is not the correct explanation for STATEMENT - 1.

Conclusion:

STATEMENT - 1 is True, STATEMENT - 2 is True; but STATEMENT - 2 is NOT a correct explanation for STATEMENT - 1. This corresponds to option B.

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