iit jee 2009

Let A be the set of all 3*3 symmetric matrices all of whose entries are either 0 or 1. Five of these entries are 1 and four of them are 0.

The number of matrices in A is (A) 12 (B) 6 (C) 9 (D) 3

Plz explain how to solve this.

2 Answers

ans is A

Fr the matrix to be symmetrical, either the diagonal elements can be -(1,0,0) or (1,1,1).

Fr the 1st case, Ways =3*3=9

Fr the 2nd case ways = 1*3 =3

Hence total no. of matrices =9+3=12.

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