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Question #40309

Difficulty: 8/10 | 🟢 Free

The Question

Let AA be a 7×77 \times 7 invertible real matrix with det⁡(A)=2\det(A) = 2, and let B=adj(A)B = \text{adj}(A) be its adjugate matrix. For any index sets X,Y⊂{1,2,…,7}X, Y \subset \{1, 2, \dots, 7\}, let MX,YM_{X, Y} denote the submatrix of a matrix MM formed by taking the rows indexed by XX and columns indexed by YY, both in increasing order. Let S={1,2,3,4}S = \{1, 2, 3, 4\} and T={1,2,4,6}T = \{1, 2, 4, 6\}. If det⁡(ATc,Sc)=5\det(A_{T^c, S^c}) = 5 and det⁡(ASc,Tc)=7\det(A_{S^c, T^c}) = 7, where XcX^c denotes the complement of XX in {1,2,…,7}\{1, 2, \dots, 7\}, what is the value of det⁡(BS,T)\det(B_{S, T})?

Options

  • A.

    -40

  • B.

    40

  • C.

    -56

  • D.

    56

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Monster

Ultra-high difficulty questions designed to test your limit, stamina, and deep conceptual understanding.

Concepts Behind This Question

Set Complement Boundaries|Row/Column Exchange Parity|Determinant Product Rule

Why This Question Is Tricky

Common Mistakes

This question tests the advanced application of Jacobi's Theorem for adjugate minors, specifically targeting the 'Row/Column Exchange Parity' and 'Set Complement Boundaries' traps.

Students commonly make two lethal errors here: (1) ignoring the permutation parity shift (−1)∑S+∑T(-1)^{\sum S + \sum T} caused by reordering the index boundaries, which leads to the incorrect sign (Option B), and (2) failing to transpose the complementary sets (using ASc,TcA_{S^c, T^c} instead of ATc,ScA_{T^c, S^c}), which collapses the algebraic structure and leads to evaluating the wrong submatrix (Options C and D).

Quick Facts About This Question

Question Information

SubjectMathematics
ChapterSets - Class 11
ConceptSet Complement Boundaries
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree