Question Details

If A = [ 10 2 k + 5   3 k 3 k + 5 ] is a symmetric matrix, the value of k is _______.

Options

A

8

B

5

C

-0.4

D

(1+1561)/12

Correct Answer :

8

Solution :

The correct option is 8.

To find the value of k, let us first recall the definition of a symmetric matrix. A square matrix A is symmetric if it is equal to its transpose, which means AT=A. For a 2×2 matrix of the form:
A=[a11a12a21a22]
the symmetry condition requires that the off-diagonal elements are equal, i.e., a12=a21.

Given the matrix:
A=[102k+53k3k+5]
the off-diagonal elements are:
a12=2k+5
and
a21=3k3.

Setting these two elements equal to each other gives the following linear equation:
2k+5=3k3

To solve for k, we subtract 2k from both sides of the equation:
5=k3

Next, we add 3 to both sides of the equation:
k=5+3
k=8

Thus, the value of k that makes the matrix symmetric is indeed 8.

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