Question Details

Consider a 3 ×3matrix A whose (i,j)-th element, ai,j = (i-j). Then the matrix A will be

Options

A

null.

B

symmetric.

C

skew-symmetric.

D

unitary.

Correct Answer :

skew-symmetric.

Solution :

The correct option is skew-symmetric.

To determine the nature of the matrix A, let us analyze its elements. The matrix A is a 3×3 matrix where the element in the i-th row and j-th column is given by the formula:
a i , j = ( i - j ) 3

Recall the definition of a skew-symmetric matrix. A matrix A is skew-symmetric if its transpose is equal to its negative, which means:
A T = - A
In terms of individual elements, this condition is written as:
a j , i = - a i , j
for all indices i and j.

Let us check if this property holds for the elements of matrix A:
a j , i = ( j - i ) 3

We can factor out -1 from the term inside the parenthesis:
j - i = - ( i - j )

Now, substituting this back into the expression for aj,i gives:
a j , i = [ - ( i - j ) ] 3
Since the cube of -1 is -1 (i.e., (-1)3=-1), we get:
a j , i = - ( i - j ) 3

Using the definition of ai,j, we have:
a j , i = - a i , j

Additionally, let us evaluate the diagonal elements where i=j:
a i , i = ( i - i ) 3 = 0 3 = 0
All diagonal elements are zero, which is a key property of a skew-symmetric matrix.

Since the relation aj,i=-ai,j holds true for all i and j, the matrix A is skew-symmetric.

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