From factors to exponents

Resource Info

Basic Information

Factorisation is a term that sounds more complex than it actually is - and this lesson plan shows how you can make the concept accessible to young children who often struggle to understand the long strings of numbers that they see! 

Lesson plan Details

Duration: 
04 hours 00 mins
Introduction: 

We encounter factors of numbers in several situations of life. Parents use them when they try to divide things equally among children. You use them when you try to divide the class into small groups to do some activities. For instance, if, in your class there were 35 students, you might divide them into small groups so that each group has the same number of students. But what if there were 37 students? Would you be able to divide them into equal groups?

Objective: 

To use story-telling to drive home basic concepts

To introduce a fun element to enjoy learning about numbers
Steps: 

From factors to exponents

We encounter factors of numbers in several situations of life. Parents use them when they try to divide things equally among children. You use them when you try to divide the class into small groups to do some activities. For instance, if, in your class there were 35 students, you might divide them into small groups so that each group has the same number of students. But what if there were 37 students? Would you be able to divide them into equal groups?
 
Understanding Factors
This is a real example that helps in understanding factors, and children need such examples to help them when they are learning factors. Here is a little story you can use:
 
In a faraway village a group of children decided to explore their neighbourhood forest. As they wandered around the forest, they found a box hidden in a hole in a huge tamarind tree. They took the box out and found that it contained some rare coins. They quickly counted the coins and found that there were 45 of them, which they divided among themselves equally.
 
Now ask your students to guess how many children there were in the group. Of course, you will get 
different answers, as there are several possibilities.
(i) There could have been 3 children and each child could have got 15 coins.
(ii) There could have been 5 children and each child could have got 9 coins.
(iii) There could have been 9 children and each child could have got 5 coins.
(iv) There could have been 15 children and each child could have got 3 coins.
(v) Or there could have been 45 children and each child could have got 1 coin.
 
Ask them if there are any more possibilities. There are none, right, so point this out. Also tell them the 
reason why this is so: it is because the numbers 1, 3, 5, 9, 15, and 45 are the only numbers that divide 
45. These are called the divisors or the factors of 45.
 
Factorizing numbers
Now draw attention to the number 45 again. Highlight the first possibility, 45=3 x15. Point out that 15 itself can be broken into factors and written down as 3x5. So we can write 45=3x3x5. In other words, the only factors of 3 are 1 and 3 and similarly for 5, its only factors are 1 and 5. Natural numbers that have exactly two divisors, namely 1 and itself, are called prime numbers and natural numbers that are the product of at least two prime numbers are called composite numbers.
 
Point out that in the above example we wrote 45 as a product of prime numbers. This is called factorizing 45 into a product of prime numbers. We can do this for any number. For example,
 
100=2x50=2x2x25=2x2x5x5
60=2x30=2x2x15=2x2x3x5
81=9x9=3x3x3x3
210=2x105=2x3x35=2x3x5x7
 
Get the children to try and factorize all the numbers from 2 to 100.
 
Factors and multiples
 
Factorizing is one thing, but one can also do the reverse: one can build up new numbers by multiplying 
prime numbers. Provide examples to the children:
2x3=6
2x2x3x5=60
3x3x5x7=315
 
Let them have some fun computing such combinations on their own. This is where you can also help 
children understand the difference (and the relationship) between factors and multiples. From the 
above examples, take the number 60 and ask the children for its factors, putting them down in a column 
on the board (1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 20, and 60). Now, in a second column put down the numbers 
that are obtained by multiplying 60 with another number. So you get 120 (60x2), 180 (60x3), 240, 300, 
and so on —explain that these numbers are multiples of 60. Let them try this with smaller numbers. E.g.:
24 (factors 1, 2, 3, 4, 6, 8, 12, and 24; multiples 24, 48, 72, 96, 120, and so on).
 
Factors to exponents
Now that they know how to build new numbers, get them to do this using the same factors each time, but in varying numbers. Let’s say they should use the prime numbers 2, 3, 5, and 7. What if they used each of these only once? They would get the number 210. Now ask them if they can guess (or calculate) what
2 x2x3x3x3x5x5x5x5x5x7x7x7x7x7x7x7 will be.
 
The answer is a big number having 11 digits: 11117830500. The prime numbers that are the factors of 11117830500 are 2, 3, 5, and 7—the same 
numbers that are the prime factors of 210! The difference lies in how many times 2, 3, 5, and 7 appear in the factorization of these two numbers. In 210, each of them appears only once, whereas in 11117830500, the prime number 2 appears twice, 3 appears three times, and so on. 
 
Put up the equation on the board and get the children to look carefully at the factorization:
11117830500=2x2x3x3x3x5x5x5x5x5x7x7x7x7x7x7x7
 
It takes up a lot of space, doesn’t it? More importantly, to find out how many times each prime number 
appears in the factorization, we need to count them. Now if we were to write it as 11117830500=22x33x55x77, where the small number at the top right showed us how many times a number appears, wouldn’t that be a neat way of keeping track of the number of times a prime number appears and saving space too? Tell the children that we read 22, 33, etc., as 2 to the power of 2, 3 to the power of 3 and so on. So 11 to the power of 3 will be 113=11x11x11.
 
Get the children to go back with this new notation and rewrite the factorization of some numbers:
45= 32x5
100=22x52
60=22x3x5
81=34
 
Using the above examples, you can get children to understand that prime numbers are the building blocks from which other numbers are made using repeated multiplication. Therefore, they are very important in mathematics. The fact that almost every number can be factored as a product of prime numbers as we have done here was known to the Greeks even before 300 BC, i.e., 300 years before the birth of Jesus Christ, more than 2300 years ago! Even ancient Greek mathematicians like Pythagoras and Euclid were fascinated by prime numbers and the factorization of a composite number into its prime factors

 

Assessment: 

 

 

Personal Reflection: 

The ‘Sieve of Eratosthenes’

Can the children guess how many prime numbers there are? Now that’s a difficult question! So start with a simple one: how many prime numbers do they know? Take them over the definition of a prime number given above once again, and point out that 1 cannot be a prime number because it has only one divisor, namely 1 itself. 2 is a prime number – the smallest prime. 3, 5, 7, 11, 13, 17, and 19 are some of the prime numbers that may be easily identified. But can they list all the prime numbers below 100? How can they find out whether a number is prime or not? There really is no formula! But there is a method called the ‘Sieve of Eratosthenes’. A sieve is something that we use to sift. We use sieves of different kinds to sift the un-powdered grains from the wheat flour, to filter tea leaves from tea, to drain out the excess water in cooked rice and so on. Sometime around 200 BC, Eratosthenes, another Greek mathematician, devised a method by which prime numbers could be filtered from other numbers.
 
This is how it works:
First we make a table of all the numbers less than 100. On the table we cross out 1 because it is not a prime number. Then we circle 2 and cross out all the multiples of 2. What is the smallest number in the table that is not circled or crossed out? 3, is it not? Now we circle 3 and cross out all the multiples of 3. 
We proceed this way till all the numbers are either circled or crossed out. The circled numbers are the prime numbers less than 100. (If you give children a little time to look at the table carefully, they will realize that 2 is the only even prime number.)
 
Now ask if the children can think of a prime number bigger than 97. It will have to be bigger than 100, of course. Perhaps they can try and extend the ‘Sieve of Eratosthenes’ up to 150 and list all the prime numbers less than 150. There are several interesting conjectures about prime numbers. Tell the children that a conjecture is a statement we believe to be true but do not know for sure whether it is true or not. 
One is that every even number greater than 2 can be written as a sum of two primes.
For example,
4= 2+2,
6=3+3,
8=3+5.
 
Try and write the following even numbers as sum of two prime numbers: 22, 34, 46, 58, and 60. 
 
Another is that every even number is a difference of two consecutive prime numbers, that is, prime numbers that are next to each other like 3 and 5, or 19 and 23 and so on. So 2=5-3 and 4=23-19 and so on. Can the children write 6, 8, 10, 12 and 14 as the difference of two consecutive primes?
 
Inspire the children saying, “If you find a number for which these conjectures are not true, you will become a very famous person!” And if you come across some interesting results about factors, multiples, and prime numbers do share them with us! 
 
From factors to exponents | Maths | April 2011 Page 6 of 6
www.teachersofindia.org | Azim Premji Foundation | www.teacherplus.org
This article first appeared in Teacher Plus, Vol. IV, Issue No 1, January-February 2006 and has been adapted here with changes.

Comments

sbabu01's picture

A needy lesson plan

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