Prove Theorem \(2.2(\mathrm{i}):(A B) C=A(B C)\)
Let \(A=\left[a_{i j}\right], \quad B=\left[b_{j k}\right], \quad C=\left[c_{k
i}\right], \quad\) and let \(A B=S=\left[s_{i k}\right], \quad B C=T=\left[t_{j
l}\right] .\) Then
$$
s_{i k}=\sum_{j=1}^{m} a_{i j} b_{j k} \quad \text { and } \quad t_{j
l}=\sum_{k=1}^{n} b_{j k} c_{k l}
$$
Multiplying \(S=A B\) by \(C\), the il-entry of \((A B) C\) is
$$
s_{i 1} c_{1 l}+s_{i 2} c_{2 l}+\cdots+s_{i n} c_{n l}=\sum_{k=1}^{n} s_{i k}
c_{k l}=\sum_{k=1}^{n} \sum_{j=1}^{m}\left(a_{i j} b_{j k}\right) c_{k l}
$$
On the other hand, multiplying \(A\) by \(T=B C\), the il-entry of \(A(B C)\) is
$$
a_{i 1} t_{1 l}+a_{i 2} t_{2 l}+\cdots+a_{i m} t_{m l}=\sum_{j=1}^{m} a_{i j}
t_{j l}=\sum_{j=1}^{m} \sum_{k=1}^{n} a_{i j}\left(b_{j k} c_{k l}\right)
$$
The above sums are equal; that is, corresponding elements in \((A B) C\) and
\(A(B C)\) are equal. Thus, \((A B) C=A(B C)\)