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# Basics of Matrices

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 1 Introduction 2 What is a Matrix? 3 Addition of Matrices 4 Subtraction of Matrices 5 Multiplication of Matrices 6 Example 7 Summary

September 8, 2020,

## Introduction

This article talks about Matrices, how they are represented and the simple operations that can be performed on Matrices. We will also see a real-time example where a Matrix can be used effectively to solve a mathematical problem statement.

## What is a Matrix?

To put it simply, Matrix is a rectangular arrangement of rows and columns. Numbers, symbols, shapes, objects, etc. can be arranged in rows and columns as shown here.  They are called Elements.
The plural of Matrix is Matrices.

### Notations

A matrix is always expressed as a capital letter, like A, B, and so on... The elements inside a matrix are denoted by lower case letters along with subscripts referring to the row and column numbers.

A row is horizontal and runs from left to right
A column is vertical and runs from top to bottom

An example notation is shown below

a1,1 refers to the element in the first row and the first column, a1,2 refers to the element in first row and second column, and so on.

### Dimensions

The dimension of a matrix is determined by the number of columns and rows it has. Number of rows are mentioned first, followed by the number of columns.
For instance, in the following matrix, there are 2 rows and 4 columns.
So, the dimension/size of this matrix is 2 × 3 (read as 2 by 3)

The number of elements in all the rows within the matrix must be the same. Similarly, the number of elements in all the columns must also be the same.
Let us see some examples here.

The figure shown above does not represent a valid matrix, as there are only 2 elements in the first row while there are 3 elements in the second row.
Similarly, if you watch closely, the number of elements in the first column and the second column is 2. But the number of elements in the third column is only 1. So, this is not a valid matrix.

## Addition of Matrices

We can perform an addition operation on 2 Matrices, just like we add 2 numbers. We will see how it is done.
If two matrices are to be added, the dimensions (sizes) of both the matrices must be exactly the same. For example, we can add two 2 X 3 matrices. But not one 2 X 4 and one 2 X 2.

Let’s see an example of the addition of 2 similar-sized matrices,

Matrix addition is quite simple. We just need to add the elements in specific positions in Matrix 1 to the elements in the (same) corresponding positions in Matrix 2. In the example shown above, we need to add the number 2 in the first Matrix to the number 4 in the second Matrix and place the answer in the same position in the resultant Matrix.
Let us see how the resultant matrix looks like,

Isn’t this simple?
Yes. But we have to remember this basic rule before carrying out the addition operation.

## Subtraction of Matrices

Subtraction is similar to the addition operation.

Numbers in respective positions are subtracted and the answer placed in the same place. So, the answer will look like what is shown below.

## Multiplication of Matrices

We can also carry out the multiplication of Matrices.
We will see the steps below.

### Scalar Multiplication of a Matrix

First, let us see how to multiply a single number (constant) to a matrix.

We need to multiply the constant (number) to every element inside the matrix and then we get the answer as given below.

Multiplying a matrix by a constant number is called Scalar Multiplication.

Okay. But Is it possible to multiply two matrices?
The answer is yes. We need to perform Dot Product of rows and columns in the matrices.

### Dot Product of Matrices

Step 1:  Multiply the first element of the first row in the first matrix by the first element of the first column in the second matrix. Repeat a similar logic and multiply all the elements in the first row to the elements in the second column of the second Matrix.
And repeat all these steps for the second row in the first Matrix.

Step 2: Add the products in each category and put the sums in the respective positions in the resultant matrix.
For example, for First row: First Column
(2 × 3 = 6) + (6 × 7 = 42) + (9 × 1 = 9)

The answer 57 is written as shown below. The same process is repeated for all categories.

When we go through the steps listed above, it is very clear that the first matrix and the second matrix need to be in a specific format for carrying out the multiplication operation.

So, what is the pre-requisite for multiplying two matrices?

Next, we will see an example of - how Matrices can be used for a practical mathematical problem.

## Example

Three friends Rohan, Sam, and Neha scored the marks given below in the two consecutive tests in their schools.

 Name Math Science English History Test 1 Rohan 97 90 93 88 Sam 99 100 90 80 Neha 100 80 88 79 Test 2 Rohan 87 89 78 90 Sam 96 80 72 80 Neha 92 99 90 76

They wanted to check their total scores in each subject in the last 2 tests.
We can use Matrices to solve this quickly. Let us see how!
Let us begin, by listing the marks scored in the first test in the form a simple Matrix.

Test 1

Similarly, let us list down the marks scored in the second test.

Now, based on the method explained in this article, we will add Test 1 and Test 2 matrices to find the total score for each student in both the tests.

Which gives us,

We now have the total scores for all three students in each of the 4 subjects.

## Summary

In this article, we learned about Matrices, how they are represented, and how simple mathematical operations like Addition, subtraction, and multiplication can be performed on them.
We will continue to learn more about the properties and rules of Matrices in a more Advanced Matrices article, Matrices and Determinants.

Written by Gayathri Sivasubramanian, Cuemath Teacher

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