BASIC MATHEMATICS FORM TWO-TOPIC: 10: SETS

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TOPIC 10: SETS

Sets

Forget everything you know about number and forget that you even know what a
number is. This is where mathematics starts. Instead of mathematics with
numbers we will think about mathematics with things.The
word set means collection of related things or objects. Or, things
grouped together with a certain property in common. For example, the
items you wear: shoes, socks, hat, shirt, pants and so on. This is
called a set. A set notation is simple, we just list each element or
member (element and member are the same thing), separated by comma, and
then put some curly brackets around the whole thing. See an example
below:

The Curly brackets are sometimes called “set brackets” or “Braces”.Sets are named by capital letters. For example; A = {1, 2, 3, 4 …} and not a = {1, 2, 3, 4, …}.To show that a certain item belongs to a certain set we use the symbol .For example if set A = {1, 2, 3, 4} and we want to show that 1 belongs to set A (is an element of set A) we write 1∈A.To
show the total number of elements that are in a given set, say set A,
we use the symbol n(A). Using our example A = {1, 2, 3, 4}, then, the
total number of elements of set A is 4. Symbolically , we write n(A) = 4Description of a SetA SetDefine a setWe
describe sets either by using words, by listing or by Formula. For
example if set A is a set of even numbers, we can describe it as
follows:

  1. By using words: A = {even numbers}
  2. By listing: A = {2, 4, 6, 8, 10,…}
  3. By Formula: A = {x: x = 2n, where n = 1,2,3,…} and is read as A is a set of all x such that x is an even number.

Example 1Describe the following set by listing: N is a set of Natural numbers between 0 and 11SolutionN = {1,2,3,4,5,6,7,8,9,10}Example 2Write the following named set using the formula: O is a set of Odd numbers:SolutionO = {x: x = 2n – 1, whereby n = 1,2,3….}Example 3Write the following set in words: W = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}SolutionW = {whole numbers} or W is a set of whole numbers.The Members of a SetList the members of a setThe objects in a set are called the members of the set or the elements of the set.A set should satisfy the following:

  1. The members of the set should be distinct. (not be repeated)
  2. The members of the set should be well-defined. (well-explained)

Example 4In question 1 to 3 list the elements of the named sets.

  1. A={x: x is an odd number <10}
  2. B={days of the week which begin with letter S}
  3. C={prime numbers less than 13}

Solution1. A={1,3,5,7,9}2. B={Saturday,Sunday}3. C={2,3,5,7,11}Naming a SetName a setTo describe a small set, we list its members between curly brackets {, }:

  • {2, 4, 6, 8}
  • { England, France, Iran, Singapore, New Zealand }
  • { David Beckham } {}
  • (the empty set, also written ∅)

We write a ∈ X to express that a is a member of the set X. For example 4 ∈ {2, 4, 6, 8}. a /∈ X means a is not a member of X.Differentiate Sets by Listing and by Stating the MembersDistinguish sets by listing and by stating the membersBy Stating the members: A = {even numbers}By listing: A = {2, 4, 6, 8, 10,…}Types of SetsA Universal Set and an Empty SetDefine a universal set and an empty setUniversal setThis is a set that contains everything that we are interested in. The symbol for universal set is μor
U. for example, the set of Integers contains all the elements of sets
such as odd numbers, prime numbers, even numbers, counting numbers and
whole numbers. In this example the set of integers is the Universal set.Another
example of a Universal set is a Set of all English Alphabets which
contains all elements of a set of vowels and set of Consonants.Empty set or Null set: is a set with no elements. There aren’t any elements in it. Not one. Zero elements. For example; A set of Countries South of the South Pole.It is represented by Øor {}.The Difference Between Finite and Infinite SetsDistinguish between finite and infinite setsFinite sets: is a set which its elements can be counted.
We can say how many members are there. For example; a set B is a set of
numbers between 1 and 7. When we list the elements, then set B =
{2,3,4,5,6}. So, there 5 elements. This set is called finite set.Infinite set:
this is a set whereby we cannot count the number of elements of the
set. We cannot tell how many members are there in a set. For example; A
is a set of all real numbers. Real numbers are all positive and negative
numbers including fractions. We cannot count the members of a set of
real numbers. Another example; B = {1,2,3,…}. Three dots means go on or
infinite, we will go on with no end. This types of sets are called
infinite sets.The Difference Between Equivalent and Equal SetsDistinguish between equivalent and equal setsEquivalent sets:
Two sets are said to be equivalent if their members match exactly. For
example; if A = {a, b, c, d} and B = {w, x, y, z} the two sets match
like this:

Generally, two sets are equivalent if n(A) = n(B). Symbolically we write A ≡B which means A is equivalent to B.Equal sets:
If two sets are equivalent and their members are alike, then the two
sets are said to be equal. For example; if A = {a, b, c, d} and B = {c,
a, b, d} then the two sets are equal since a is in set A and in set B, b
is in set A and in set B, c is in set A and in set B and d is in set A
and in B. Also, numbers of elements of the both sets are equal.
Therefore A = B (set A is equal to set B)SubsetsA SubsetDefine a subsetWhen we define a set, if we take piece of that set, we can form what is called a subset. For
example; if we have a set {a, b, c, d, e}, a subset of this is {b, c,
d}. Another subset is {a, b} or even another subset is {e} or {d} and so
on. However {a, f} is not a subset since it contains an element (f)
which it is not in the parent set.Generally, A is a subset of B if and only if every element of A is in B. symbolically we write A⊂B (means A is a subset of B).Subsets of a Given SetList subsets of a given setFor
example; if A = {1, 2, 3, 4}, B = {1, 2, 3} and C = {1, 2, 3, 4} then, B
is a proper subset of A i. e. B⊂A and C is an improper subset of A i.e.
C⊆A.Important note: an Empty set is a subset of any set.The Difference between Proper and Improper SubsetsDistinguish between proper and improper subsetsIf every element in A is also in B, and there exist at least one element in B that is not in A, we say that A is Proper subset of B.And if every element in A is in B, and there is no element in B that is not in A, we say that A is an improper subset of B and we write A = B or symbolically we write A⊆B or B⊆A.The Number of Subsets in a SetCalculate the number of subsets in a setConsider an example below:

When
you look at the table, you will see that the number of subsets can be
obtained by 2 raised to the number of elements of the set under
consideration. Therefore, the formula for finding the number of subsets
of a set with n elements is given by 2n, n is a number of elements of a set.Example: How many subsets are there in set A = {Red, White, Yellow}. List them.Solution: case 1, number of subsetsSet A has 3 elements. ButNumber of subsets = 2n, so number of subsets of set A = 23 = 8Therefore set A has 8 subsetsCase 2: list of subsets.The subsets of set A are: { },{Red},{White},{Yellow},{Red,White},{Red,Yellow},{White,Yellow}, {Red,White,Yellow}.Operations With SetsUnion of Two SetsFind union of two setsWhen
elements of two or more sets are put together with no repetition, we
get another set which is a union set. The symbol for union is ∪.For example; if A = {a,b} and B = {a,b,c,d,e}, then A∪B = {a,b,c,d,e}.Another example; if A = {a,b,c} and B = {5,6,2,4}, find A∪B.SolutionA∪B = {a,b,c,5,6,2,4}The Compliment of a SetFind the compliment of a setComplement means ‘everything that is not’. For example; if A is a subset of a universal set, the elements of a universal set that are not in A are the complements of set A. complement of a set is denoted by C. So complement of set A is written as Ac. Or A′.For example; if ∪= {0,1,2,3,4,5,6,7,8,9,} and A = {1,3,5,7,9}. Find Ac.SolutionWe are required to find elements which are not in A but are in ∪.Therefore, Ac= {2,4,6,8}.The Number of Elements in the Union and Intersection of two SetsFind the number of elements in the union and intersection of two setsIf A = {a,b,c} and B = {5,6,2,4}, find A∪B.SolutionA∪B = {a,b,c,5,6,2,4}IntersectionIf
we have two sets A and B and we decide to form a new set by taking only
common elements from both sets i.e. elements which are found both in A
and B. This new set is called intersection of set A and B. the symbol
for intersection is ∩. Intersection of sets A and B is denoted by A∩B.
for example; if A = {a,b,d,e} and B = {a b,d,f,g} then, the common
elements are: a, b and d. Therefore, A∩B = {a,b,d}. Another example; if A
= {1,3,5,7,9} and B = {2,4,6,8}. Find A∩B.AnswerWhen
you take a look at our sets, you will notice that there is no even a
single element which is in common. Therefore, intersection of set A and B
is an Empty set i.e. A∩B = or { }.Venn DiagramsReprsenting Sets by using Venn DiagramsRepresent a sets by using venn diagramsThe
diagrams are oval shaped. They we named after John venn, an English
Mathematician who introduced them. For example A = {1,2,3} in venn
diagram can be represented as follows:

is a universal set which can be a set of counting numbers and A is a subset of it.If
we have two sets, say Set A and B and these sets have some elements in
common and we are supposed to represent them in venn diagrams, their
ovals will overlap. For example if A = {a,b,c,d,e,} and B = {a,e,i,o,u}
in venn diagrams they will look like this:

If
the two sets have no elements in common, then the ovals will be
separate. For example; if A = {1,2,3} and B = {5,6}. In venn diagram
they will appear like here below:

If
we have two sets, A and B and set A is a subset of set B then the oval
for set A will be inside the oval of set B. for example; if A = {b,c}
and B = {a,b,c,d} then in venn diagram it will look like this:

If
we have to represent the union or intersection of two or more sets
using venn diagrams, the appearance of the venn diagrams will depend on
whether the sets under consideration have some elements in common or
not.Information from Venn DiagramsInterpret information from venn diagramsCase 1: sets with elements in common.Example 1: If A = {5,6,7,9,10} and B = {3,4,7,9,11} represent A union B and A intersection B in venn diagrams.SolutionCase 1: A union B

Case 2: A intersection B

Example 2: A = {a,b,c,d,e,f}, B = {a,e} and C = {b,c,e,d}. Represent in venn diagrams A∪B∪C and A∩B ∩C.Solution: case 1. A∪B∪C

Case 2: A ∩B∩C

Case 2: sets with no elements in common:For example; A = {a,b,c,}, B = {d,f}, C = {h,g} on venn diagram will appear like this:

Number of elements in two sets say set A and B i.e. n(A∪B) is given by:n(A∪B) = n(A) + n(B) – n(A ∩B)proof: consider the venn diagram below:

From our venn diagram:n(A) = x + y, n(B) = y + z, n(A∩B) = y and n(A∪B) = x + y + z thus;n(A) + n(B) = (x + y) + (y + z)= (x + y + z) + ybut x + y + z = n(A ∪B) and n(A∩B) = yso,n(A) + n(B) = n(A∪B) + n(A∩B)make n(A∪B) be the subject of the formulaFor example; if n(A) = 15, n(A∩B) = 3 and n(A∪B) = 24. Find n(B)Soln;Recall that: n(A∪B) = n(A) + n(B) – n(A∩B)n(B) = n(A∪B) + n(A∩B) – n(A)= 24 + 3 – 15n(B) = 12Therefore, n(B) = 12Word problemsFor
example; at Mtakuja primary school there are 180 pupils. If 120 pupils
like one of the sports, either netball or football and 50 pupils likes
netball while 30 pupils likes both netball and football. How many pupils

  1. likes football.
  2. Likes neither of the sport

Solutions.Let be the universal setN be the set of pupils who likes netballF be the set of pupils who likes footballThus,n(F) = ?n(N∩F) = 30n(N∪F) = 120n(μ) = 180But we know that n(N∪F) = n(N) + n(F) – n(N∩F)Thus, n(F) = n(N∪F) + n(N∩F) – n(N)= 120 + 30 – 50n(F) = 100Therefore there are 100 pupils who likes football.2. We have a total of 180 pupils at Mtakuja primary schoolBut only 120 pupils likes one of either the sport. so, those who likes neither of the sport will be 180 – 120 = 50Therefore 50 pupils likes neither of the sport.Alternatively: by using venn diagram

n(F) only = 120 – 30 -20 = 70n(F) = those who likes both netball and football + those who likes football onlyn(F) = 30 + 70 = 100Therefore, there 100 pupils who likes football.Exercise 11. If A = {Red,White,Blue} show by using symbol that Red, White and Blue are members of set A.2. List the elements of set B if B is a set of counting numbers.3. Which of the following sets are finite, infinite or empty sets.

  1. A = {y:y is an odd number}
  2. B = {1,3,7,…35}
  3. C = { }
  4. D = {Maths,Biology,Physics,Chemistry}
  5. E = {Prime numbers between 31 and 37}
  6. F = {….-2,-1,0,1,2,…}

4. If A = {1,4,9,16,25,36}, B = {1,4,9} and C = {1,3,4}, which of the following statement is true:

  1. A⊂B
  2. B⊂A
  3. A⊆C
  4. C⊆B

5. How many subsets are there in set A = {f,g,I,k,m,n}? List them all.6.If A = {all letters of English Alphabets} and B = {c,d,g,h}.List the elements of B′.7. Let B be a set of whole numbers and C a set of prime numbers found in a set of whole numbers, using venn diagram show B∩C.8. Draw a venn Diagram and show by shading the required region:

9. If n(A) = 90, n(B) = 120 and n(A∩B) = 45. Find:

  1. n(A∪ B)
  2. n(B) only.
  3. n(A) only.

10.
In a certain meeting 40 people drank juice, 25 drank soda and 20 drank
both juice and soda. How many people were in the meeting, assuming that
each person took juice or soda?

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