# Chapter 8 - Section 8.3 - Matrix Operations and Their Applications - Exercise Set - Page 919: 74

The number of rows in the product of two matrices AB is the number of rows of matrix A and the number of columns in AB is the number of columns of matrix B.

#### Work Step by Step

The order of a matrix obtained by the product of two matrices is defined as follows: Consider a matrix A of $m\times n$ order and another matrix B of $n\times k$ order. To find the ${{i}^{th}}$ row and ${{j}^{th}}$ column of the product AB, each element in the ${{i}^{th}}$ row of the matrix A is multiplied by the corresponding element in the ${{j}^{th}}$ column of the matrix B, and then obtained products of corresponding elements are added. The product matrix AB is an $m\times k$ order matrix, that is, if $A={{\left[ {{a}_{ij}} \right]}_{m\times n}}$ and $B={{\left[ {{b}_{jk}} \right]}_{n\times k}}$, then: Consider $AB=C$ Therefore, $C={{\left[ {{c}_{ik}} \right]}_{m\times k}}$ The order of matrix C is $\text{number of rows of matrix }A\times \text{number of columns of matrix }B$ Here, ${{c}_{ik}}=\sum\limits_{j=1}^{n}{{{a}_{ij}}{{b}_{jk}}}$ Let $A={{\left[ \begin{matrix} a & b \\ c & d \\ \end{matrix} \right]}_{2\times 2}}$ and $X={{\left[ \begin{matrix} x \\ y \\ \end{matrix} \right]}_{2\times 1}}$ Then: Calculate AX: \begin{align} & AX=\left[ \begin{matrix} a & b \\ c & d \\ \end{matrix} \right]\left[ \begin{matrix} x \\ y \\ \end{matrix} \right] \\ & =\left[ \begin{matrix} ax+by \\ cx+dy \\ \end{matrix} \right] \end{align} In the obtained matrix, the number of rows is two and the number of columns is one. Hence, the order of matrix AX is $2\times 1$.

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