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Prime numbers Arithmetic Factors

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Here we will learn about prime factors including how to express a number as a product of prime factors and use the product of prime factors to recognise special numbers such as square numbers and cube numbers.

There are also prime factors worksheets based on Edexcel, AQA and OCR exam questions, along with further guidance on where to go next if you’re still stuck.

Prime factors are prime numbers that are factors of another number.

E.g.

A composite number is the product of two or more factors. All of these types of numbers are integers (whole numbers).

In number theory, the Fundamental Theorem of Arithmetic states that every integer greater than one is a prime number or can be represented by a product of prime numbers.

E.g.

This would mean that the number

Expressing a composite number as a product of prime factors can be utilised for a wide variety of problems such as:

and much more.

We can also manipulate the prime factorisation of a number to find other numbers.

To find the prime factors of a number, we need to continue to divide the composite number by prime numbers until we are left with just prime factors.

Looking back at the example of

Here, the divisor is

Any positive integer can be written as a product of its prime factors. This means that we can take any positive number and write it as a series of prime numbers being multiplied.

E.g.

6 is a product of 2 and 3, so can be written as

9 is a product of 3 and 3, so can be written as

We can use a process called prime factor decomposition using prime factor trees in order to work out the product of prime factors.

E.g.

Write 36 as a product of prime factors.

So,

A question may ask you to give your answer in index form. To do this we write the solution using powers, so 36 = 2^{2}×3^{2}

E.g.

Write 54 as a product of prime factors. Give your answer in index form.

So,

In index form,

54=2×3^{2}

When finding prime factors it is useful to use the divisibility rules:

In order to find prime factors of a composite number:

We can also use factor trees to help us to work out the prime factors of a number.

Get your free prime factors worksheet of 20+ questions and answers. Includes reasoning and applied questions.

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Get your free prime factors worksheet of 20+ questions and answers. Includes reasoning and applied questions.

DOWNLOAD FREE

Prime factors is part of our series of lessons to support revision on factors, multiples and primes. You may find it helpful to start with the main factors, multiples and primes lesson for a summary of what to expect, or use the step by step guides below for further detail on individual topics. Other lessons in this series include:

What are the prime factors of

As

The first prime factor of

2Divide the answer by another suitable prime number

The second prime factor is

3Continue until the final answer is a prime number, then state the solution

As factors are multiplied together to achieve the composite number, we can say

(or in exponent form)

The final solution is therefore

Show that

Divide the composite number by a suitable prime number

As

The first prime factor of

Divide the answer by another suitable prime number

The second prime factor is

Continue until the final answer is a prime number, then state the solution

As factors are multiplied together to achieve the composite number, we can say

An exponent form does not occur for this example as there are no powers we can simplify.

The final solution is therefore

Express

Divide the composite number by a suitable prime number

As

The first prime factor of

Divide the answer by another suitable prime number

The second prime factor is

Continue until the final answer is a prime number, then state the solution

As factors are multiplied together to achieve the composite number, we can say

The final solution in index form is therefore

Express

Divide the composite number by a suitable prime number

As

The first prime factor of

Divide the answer by another suitable prime number

The sum of the digits of

The second prime factor is

Continue until the final answer is a prime number, then state the solution

As

The third prime factor is

As factors are multiplied together to achieve the composite number, we can say

The final solution in index form is therefore

Show that

Divide the composite number by a suitable prime number

As the sum of the digits of

The first prime factor of

Divide the answer by another suitable prime number

The sum of the digits of

The second prime factor is

Continue until the final answer is a prime number, then state the solution

As

The third prime factor is

The fourth prime factor is

The final two prime factors are

As factors are multiplied together to achieve the composite number, we can say

So

What are the prime factors of

Divide the composite number by a suitable prime number

As

The first prime factor of

Divide the answer by another suitable prime number

The sum of the digits of

The second prime factor is

Continue until the final answer is a prime number, then state the solution

As the sum of the digits of

The third prime factor is

The fourth prime factor is

As factors are multiplied together to achieve the composite number, we can say

(in exponent form)

The final solution in index form is therefore

Given example

It is easy to find a factor of a composite number and not specifically a prime factor. Take example

When we look at larger composite numbers, there is a much more clear and efficient method to expressing the prime factors of a number (see the next lesson on factor trees).

1. What is 16 as a product of its prime factors?

8 \div 2 = 4

4 \div 2 = 2

2 is prime so we have found the prime factors.

16=2 \times 2 \times 2 \times 2=2^{4}

2. Express 63 as a product of its prime factors.

21 \div 3 =7

7 is prime.

63=3 \times 3 \times 7=3^{2} \times 7

3. Express the number 246 as a product of prime factors.

123 \div 3 =41

41 is prime.

246=2 \times 3 \times 41

4. A number written as the product of its prime factors is 2 \times 3 \times 5^{2} . What is the number?

5. Which of the following does not show 36 as a product of its prime factors?

They all show 36 as a product of its prime factors

Although 2^{2} \times 9=36

9 is not a prime number.

6. 720 can be expressed in the form 2^a \times 3^b \times c . What are the values of a, b and c ?

360 \div 2 =180

180 \div 2=90

90 \div 2=45

45 \div 3 =15

15 \div 3=5

5 is prime.

720=2^{4} \times 3^{2} \times 5

a=4,\; b=2, c=5 \;

1. Express 126 as a product of prime factors. Write your answer in index form.

(3 marks)

(1)

126=2 \times 3 \times 3 \times 7

(1)

126=2 \times 3^{2} \times 7

(1)

2. (a) Express the value 456 in the form 456=\mathrm{a}^{3} \times 3 \times \mathrm{b} where a and b are prime factors. State the values of a and b in your answer.

(b) Use your answer to part (a) to write the product of primes for the value of 456 × 9 .

(5 marks)

(a)

456 \div 2=228,228 \div 2=114,114 \div 2=57,57 \div 3=19

(1)

2 \times 2 \times 2 \times 3 \times 19 \text { or } 456=2^{3} \times 3 \times 19

(1)

a=2, b=19

(1)

(b)

9=3 \times 3 \text { or } 3^{2}

(1)

456 \times 9=2^{3} \times 3 \times 19 \times 3 \times 3=2^{3} \times 3^{3} \times 19

(1)

3. (a) Which of the following options represents the prime factors of 28

\quad \quad 7 × 4 \quad \quad \quad \quad \quad 2^{2} \times 7 \\\\ 1, 2, 4, 7, 14, 28, \quad \quad \quad 2, 7

(b) What is the smallest number that you can multiply 28 by to get a square number?

(4 marks)

(a)

2^{2} \times 7

(1)

(b)

\text { Increase the power of } 7 \text { by } 1

(1)

196 \text { or } 2^{2} \times 7 \times 7 \text { implied }

(1)

7

(1)

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