If A, B, and C arc three square matrices of the same order, then AB=AC⇒B=C. Then
|A|≠0
A is invertible
A may be orthogonal
A is symmetric
If |A|≠0, then
AB = AC
or A−1AB=A−1AC
or B = C
Also, if A is orthogonal matrix, then, AAT = I
⇒ AAT=1⇒|A|2=1⇒A is invertible