A matrix A is skew-symmetric if AT=−A.
When transposing a skew-symmetric matrix, the result is the negative of the original matrix.
For a skew-symmetric matrix A raised to power n:
(An)T=(−A)n
(An)T=(−1)n×An
When n is odd: (An)T=−An (skew-symmetric)
When n is even: (An)T=An (symmetric)
For A5+B7:
A5 has odd power, so A5 is skew-symmetric.
B7 has odd power, so B7 is skew-symmetric.
The sum of two skew-symmetric matrices is skew-symmetric.
For A21:
A21 has odd power, so A21 is skew-symmetric.
For B18:
B18 has even power, so B18 is symmetric.
For A6+B7:
A6 has even power, so A6 is symmetric.
B7 has odd power, so B7 is skew-symmetric.
The sum of a symmetric matrix and a skew-symmetric matrix is neither symmetric nor skew-symmetric.
Therefore, the statement that A6+B7 is symmetric is not true.