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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

  1. A 12
  2. B 6
  3. C 9
  4. D 3
Verified Solution

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 \(=3+9=12\)
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