If A=31-12 then,
A2+7A-5I=0
A2-7A+5I=0
A2+5A-7I=0
A2-5A+7I=0
The explanation of the correct option :
Given A=31-12
A2=31-1231-12=85-53
5A=531-12=155-510
7I=71001=7007
Now, A2-5A+7I=85-53-155-510+7007
=8-15+75-5-5+53-10+7=0000=0
Hence, the correct option is (D).
If Show that A + A′ is a symmetric matrix