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# Functions And Relations

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### Functions And Relations

1. 1. Functions and Relations
2. 2. Set Notation <ul><li>A set is a collection of objects e.g A = {3,4}. </li></ul><ul><li>The objects in the set are known as the elements or members of the set. </li></ul><ul><li>For example, you are ‘elements’ of our class ‘set’. </li></ul><ul><li>3 ∈ A means ‘3 is a member of set A ’ or ‘3 belongs to A ’. </li></ul><ul><li>6 ∉ A means ‘6 is not an element of A’ . </li></ul>
3. 3. Set Notation <ul><li>If x ∈ B implies x ∈ A, then B is a subset of A , we write B ⊆ A . This expression can also be read as ‘ B is contained in A ’ or ‘ A contains B ’. </li></ul><ul><li>The set ∅ is called the empty set or null set. </li></ul><ul><li>A ∩ B is called the i n tersection of A and B . Thus x ∈ A ∩ B if and only if x ∈ A and x ∈ B . </li></ul><ul><li>A ∩ B = ∅ if the sets A and B have no elements in common. </li></ul><ul><li>A ∪ B , is the u nion of A and B. If elements are in both A and B they are only included in the union once. </li></ul><ul><li>The set difference of two sets A and B is denoted A B ( A but not B ) </li></ul><ul><li>Example 1 : A = {1, 2, 3, 7}; B = {3, 4, 5, 6, 7} </li></ul><ul><li>Find: a) A ∩ B b) A ∪ B c) A B d) B A </li></ul><ul><li>Solution: a) A ∩ B = {3, 7} </li></ul><ul><li>b) A ∪ B = {1, 2, 3, 4, 5, 6, 7} </li></ul><ul><li>c) A B = {1, 2} </li></ul><ul><li>d) B A = {4, 5, 6} </li></ul>
4. 4. Sets of numbers <ul><li>N: Natural numbers {1, 2, 3, 4, . . .} </li></ul><ul><li>Z: Integers {. . . ,−2,−1, 0, 1, 2, . . .} </li></ul><ul><li>Q: Rational numbers – can be written as a fraction. Each rational number may be written as a terminating or recurring decimal. </li></ul><ul><li>The real numbers that are not rational numbers are called irrational (e.g. π and √2). </li></ul><ul><li>R: Real numbers. (How can a number not be real? – We’ll find out later) </li></ul><ul><li>It is clear that N ⊆ Z ⊆ Q ⊆ R and this may be represented by the diagram: </li></ul>
5. 5. Sets of numbers <ul><li>The following are also subsets of the real numbers for which there are special notations: </li></ul><ul><li>R + = { x : x > 0} </li></ul><ul><li>R − = { x : x < 0} </li></ul><ul><li>R {0} is the set of real numbers excluding 0. </li></ul><ul><li>Z + = { x : x ∈ Z, x > 0} </li></ul><ul><li>Note: </li></ul><ul><li>{ x : 0 < x < 1} is the set of all real numbers between 0 and 1. </li></ul><ul><li>{ x : x > 0 , x rational} is the set of all positive rational numbers . </li></ul><ul><li>{2 n : n = 0, 1, 2, . . .} is the set of all even numbers. </li></ul>
6. 6. Representing sets of numbers on a number line <ul><li>Among the most important subsets of R are the intervals. </li></ul><ul><li>(-2, 4) means all ‘real’ numbers between (but not including) -2 and 4. </li></ul><ul><li>[3, 7] means all ‘real’ numbers between 3 and 7 inclusive. </li></ul><ul><li>[4, ∞) means all ‘real’ numbers greater than or equal to 4. </li></ul><ul><li>(-∞, 3) means all ‘real’ numbers less than 3. </li></ul>
7. 7. Representing sets of numbers on a number line <ul><li>Example 2: Illustrate each of the following intervals of the real numbers on a number line: </li></ul><ul><li>a [−2, 3] b (−3, 4] c (−∞, 5] d (−2, 4) e (−3,∞) </li></ul>
8. 8. Describing relations and functions <ul><li>An ordered pair , denoted ( x, y ), is a pair of elements x and y in which x is considered to be the first element and y the second (it doesn’t mean they have to be in numerical order). </li></ul><ul><li>A relation is a set of ordered pairs. The following are examples of relations: </li></ul><ul><li>S = {(1, 1), (1, 2), (3, 4), (5, 6)} </li></ul><ul><li>T = {(−3, 5), (4, 12), (5, 12), (7,−6)} </li></ul><ul><li>The domain of a relation S is the set of all first elements of the ordered pairs in S. </li></ul><ul><li>The range of a relation S is the set of all second elements of the ordered pairs in S. </li></ul><ul><li>In the above examples: </li></ul><ul><li>domain of S = {1, 3, 5}; range of S = {1, 2, 4, 6} </li></ul><ul><li>domain of T = {−3, 4, 5, 7}; range of T = {5, 12, −6} </li></ul><ul><li>A relation may be defined by a rule which pairs the elements in its domain and range. </li></ul><ul><li>Let’s watch an example. </li></ul>
9. 9. Describing relations and functions <ul><li>Example 3: Sketch the graph of each of the following relations and state the domain and range of each. </li></ul><ul><li>a {( x, y ): y = x 2 } </li></ul><ul><li>b {( x, y ): y ≤ x + 1} </li></ul><ul><li>c {(−2 , −1) , (−1 , −1) , (−1 , 1) , (0 , 1) , (1 , −1)} </li></ul><ul><li>d {( x, y ): x 2 + y 2 = 1} </li></ul><ul><li>e {( x, y ): 2 x + 3 y = 6 , x ≥ 0} </li></ul><ul><li>f {( x, y ): y = 2 x − 1 , x ∈ [−1 , 2]} </li></ul>
10. 10. Describing relations and functions <ul><li>A function is a relation such that no two ordered pairs of the relation have the same ﬁrst element. </li></ul><ul><li>For instance, in Example 3, a, e and f are functions but b, c and d are not. </li></ul><ul><li>Functions are usually denoted by lower case letters such as f, g, h. </li></ul><ul><li>The definition of a function tells us that for each x in the domain of f there is a unique element, y , in the range. </li></ul><ul><li>The element y is denoted by f ( x ) (read ‘ f of x ’). </li></ul>
11. 11. Describing relations and functions <ul><li>Example 4: If f ( x ) = 2 x 2 + x, find f (3) , f (−2) and f ( x − 1) . </li></ul><ul><li>Solution </li></ul><ul><li>f (3) = 2(3) 2 + 3 = 21 </li></ul><ul><li>f (−2) = 2(−2) 2 − 2 = 6 </li></ul><ul><li>f ( x − 1) = 2( x − 1) 2 + x − 1 </li></ul><ul><li>= 2( x 2 − 2 x + 1) + ( x − 1) </li></ul><ul><li>= 2 x 2 − 3 x + 1 </li></ul>
12. 12. Describing relations and functions <ul><li>Example 5: For each of the following, sketch the graph and state the range: </li></ul><ul><li>a f : [−2 , 4] -> R, f ( x ) = 2 x − 4 </li></ul><ul><li>b g : (−1 , 2] -> R, g ( x ) = x 2 </li></ul>