Sets and their Representation (Set theory part -1 NCERT)


Sets and their representations

In everyday life, we often speak of collections of objects of a particular kind, such as, a pack of cards, a crowd of people, a cricket team, etc. In mathematics also, we come across collections, for example, of natural numbers, points, prime numbers, etc. More specially, we examine the following collections: 

(i) Odd natural numbers less than 10, i.e., 1, 3, 5, 7, 9 
(ii) The rivers of India 
(iii) The vowels in the English alphabet, namely, a, e, i, o, u 
(iv) Various kinds of triangles 
(v) Prime factors of 210, namely, 2,3,5 and 7 
(vi) The solution of the equation: x2 – 5x + 6 = 0, viz, 2 and 3. 

We note that each of the above example is a well-defined collection of objects in the sense that we can definitely decide whether a given particular object belongs to a given collection or not. For example, we can say that the river Nile does not belong to the collection of rivers of India. On the other hand, the river Ganga does belong to this colleciton. 
We give below a few more examples of sets used particularly in mathematics, viz. 

N : the set of all natural numbers 
Z : the set of all integers 
Q : the set of all rational numbers 
R : the set of real numbers 
Z+ : the set of positive integers 
Q+ : the set of positive rational numbers, and 
R+ : the set of positive real numbers. 

The symbols for the special sets given above will be referred to throughout this text. 

Again the collection of five most renowned mathematicians of the world is not well-defined, because the criterion for determining a mathematician as most renowned may vary from person to person. Thus, it is not a well-defined collection. 
We shall say that a set is a well-defined collection of objects
The following points may be noted : 
(i) Objects, elements and members of a set are synonymous terms. 
(ii) Sets are usually denoted by capital letters A, B, C, X, Y, Z, etc. 
(iii) The elements of a set are represented by small letters a, b, c, x, y, z, etc. 

If a is an element of a set A, we say that “ a belongs to A” the Greek symbol ∈(epsilon) is used to denote the phrase ‘belongs to’. Thus, we write a∈A. If ‘b’ is not an element of a set A, we write b ∉A and read “b does not belong to A”. 
Thus, in the set V of vowels in the English alphabet, a ∈V but b ∉V. In the set P of prime factors of 30, 3 ∈P but 15 ∉P. 

There are two methods of representing a set : 
(i) Roster or tabular form 
(ii) Set-builder form. 

In roster form, all the elements of a set are listed, the elements are being separated by commas and are enclosed within braces { }. For example, the set of all even positive integers less than 7 is described in roster form as {2, 4, 6}. Some more examples of representing a set in roster form are given below : 
(a) The set of all natural numbers which divide 42 is {1, 2, 3, 6, 7, 14, 21, 42}. 

Note: In roster form, the order in which the elements are listed is immaterial. Thus, the above set can also be represented as {1, 3, 7, 21, 2, 6, 14, 42}. 

(b) The set of all vowels in the English alphabet is {a, e, i, o, u}. 
(c) The set of odd natural numbers is represented by {1, 3, 5, . . .}. The dots tell us that the list of odd numbers continue indefinitely. 

Note: It may be noted that while writing the set in roster form an element is not generally repeated, i.e., all the elements are taken as distinct. For example, the set of letters forming the word ‘SCHOOL’ is { S, C, H, O, L} or {H, O, L, C, S}. Here, the order of listing elements has no relevance. 

In set-builder form, all the elements of a set possess a single common property which is not possessed by any element outside the set. For example, in the set {a, e, i, o, u}, all the elements possess a common property, namely, each of them is a vowel in the English alphabet, and no other letter possess this property. Denoting this set by V, we write 
V = {x : x is a vowel in English alphabet} 

It may be observed that we describe the element of the set by using a symbol x (any other symbol like the letters y, z, etc. could be used) which is followed by a colon “ : ”. After the sign of colon, we write the characteristic property possessed by the elements of the set and then enclose the whole description within braces. The above description of the set V is read as “the set of all x such that x is a vowel of the English alphabet”. In this description the braces stand for “the set of all”, the colon stands for “such that”. For example, the set 

A = {x : x is a natural number and 3 < x < 10} is read as “the set of all x such that x is a natural number and x lies between 3 and 10. Hence, the numbers 4, 5, 6, 7, 8 and 9 are the elements of the set A. 
If we denote the sets described in (a), (b) and (c) above in roster form by A, B, C, respectively, then A, B, C can also be represented in set-builder form as follows: 
A= {x : x is a natural number which divides 42} 
B= {y : y is a vowel in the English alphabet} C= {z : z is an odd natural number} 

Example 1 Write the solution set of the equation x2 + x – 2 = 0 in roster form. 
Solution The given equation can be written as 
(x – 1) (x + 2) = 0, i. e., x = 1, – 2 
Therefore, the solution set of the given equation can be written in roster form as {1, – 2}. 

Example 2 Write the set {x : x is a positive integer and x2 < 40} in the roster form. 
Solution The required numbers are 1, 2, 3, 4, 5, 6. So, the given set in the roster form is {1, 2, 3, 4, 5, 6}. 

Example 3 Write the set A = {1, 4, 9, 16, 25, . . . }in set-builder form. 
Solution We may write the set A as 
A = {x : x is the square of a natural number} 
Alternatively, we can write 
A = {x : x = n2, where n ∈N} 

Example 4 Write the set {1/2, 2/3, 3/4, 5/6, 6/7} in the set-builder form. 
Solution We see that each member in the given set has the numerator one less than the demominator. Also, the numerator begin from 1 and do not exceed 6. 

Example 5 Match each of the set on the left described in the roster form with the same set on the right described in the set-builder form : 
(i) {P, R, I, N, C, A, L} (a) { x : x is a positive integer and is a divisor of 18} 
(ii) { 0 } (b) { x : x is an integer and x2 – 9 = 0} 
(iii) {1, 2, 3, 6, 9, 18} (c) {x : x is an integer and x + 1= 1} 
(iv) {3, –3} (d) {x : x is a letter of the word PRINCIPAL} 
Solution Since in (d), there are 9 letters in the word PRINCIPAL and two letters P and I are repeated, so (i) matches (d). Similarly, (ii) matches (c) as x + 1 = 1 implies x = 0. Also, 1, 2 ,3, 6, 9, 18 are all divisors of 18 and so (iii) matches (a). Finally, x2 – 9 = 0 implies x = 3, –3 and so (iv) matches (b).