If 
and 
verify that (A + B) + C = A + (B+C).
(A+B)+C 
A+(B+C) 
Therefore, (A+B)+C = A+(B+C)
It is true for any matrix
Conclusion: (A+B)+C = A+(B+C)
1
If and verify that (A + B) + C = A + (B+C).
(A+B)+C
A+(B+C)
Therefore, (A+B)+C = A+(B+C)
It is true for any matrix
Conclusion: (A+B)+C = A+(B+C)