A is an invertible symmetric matrix, meaning A=AT.
To determine the type of matrix A−1 is, consider the transpose of A−1.
Using the transpose-inverse property:
(AT)−1=(A−1)T
Since A=AT:
(AT)−1=(A)−1
=A−1
Combining these results:
A−1=(A−1)T
This is the definition of a symmetric matrix.
Therefore, A−1 is a symmetric matrix.