JEE Advanced · Mathematics · 4. P&C
Paragraph:
Let \(A\) be the set of all \(3 \times 3\) symmetric matrices all of whose entries are either 0 or 1 . Five of these entries are 1 and four of them are 0.
Question:
The number of matrices in \(A\) is
- A 12
- B 6
- C 9
- D 3
Answer & Solution
Correct Answer
(A) 12
Step-by-step Solution
Detailed explanation
Case \(I\) When all three diagonal elements are 1 , then
Number of matrices \(={ }^3 C_1=3\)
Case II When two diagonal elements are zero and one element is one no, then Number of matrices \(={ }^3 C_1 \cdot{ }^3 C_1=9\)
\(\therefore\) Total matrices \(=3+9=12\)
Number of matrices \(={ }^3 C_1=3\)
Case II When two diagonal elements are zero and one element is one no, then Number of matrices \(={ }^3 C_1 \cdot{ }^3 C_1=9\)
\(\therefore\) Total matrices \(=3+9=12\)
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