Chapter 1 functions

Additional Mathematics Module Form 4
Chapter 1- Functions SMK Agama Arau, Perlis

CHAPTER 1- FUNCTIONS

1.1 RELATIONS
1.1.1 Representing Relations

1- By Arrow Diagram
One to many relation
One to one relation
x x
x x+2
−2
1 3 4 2
2 4 9 3
3 5 −3

Many to many relation
Many to one relation

x x2 360 min
6 hours
1 min 3600 sec
2 60 sec
2160 sec
−2 4 0.25 day
1 hour
3 9
−3

2- By Ordered Pair

(1, 3), (2, 4), (3, 5)

(1, 4), ( − 2, 4), (3, 9), ( − 3, 9)

3. By Graph

y (Image)

4 X

3 X

0 1 2 x (object)

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Set A is called domain of the relation and set B is the codomain.
Each of the elements in the domain (Set A) is an object. So 1, 2 and 3 are called object.
Each of the elements in the codomain (Set B) is an image. So 3, 4, 5 and 6 are called image.
The set that contains all the images that are matched is the range.

domain

A B

1 3
codomain
2 4
3 5
6

Domain={ 1,2,3 } codomain= {3,4,5,6}
Object = 1, 2, 3 image = 3, 4, 5, 6
Range= {3, 4, 5}

A B
Object image

1.2 FUNCTIONS

1. Function is a special relation where every object in a domain has only one image. A function is also
known as a mapping.

2. From the types of relation we have learned, only one to one relation and many to one relation are
function.

Function notation
Small letter: f, g, h or something else…
f : x → 2x
Read as function f map x onto 2x Look at the difference between
these two.
Example:
f:x 2x Concept:
f(x) = 2x f : x → 2x
x = 1, f (x) = 2x
f(1) = 2(1)
=2 image

object
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Example 1:
If the given is object, so we are going to
1. Given function f : x → 3x − 2 . Find find the value of image. -1 is the object
because we have to find the value of its
(a) the image of –1 image.

f(x)= 3x – 2
Remember the concept:
f(-1) = 3(– 1) – 2
f:x 3x – 2 then
=–3–2
f(x)= 3x – 2 and when x= –1,
= –5
f(-1) = 3(– 1) – 2
(b) object which has the image 4

f(x) = 3x – 2 1 If the given is image, so we are going to find
Given that the image is 4, the value of object. 4 is the image because we
So f(x) = 4 2 have to find the value of its object.

Compare 1 and 2 ,
Hence,
3x – 2 = 4
3x = 6
x=2

2. Given that f(x) = px + 3 and f(4)= 5. Find the value of p.

f(x) = px + 3
If x=4,
1
f(4)= 4p + 3
f(4)= 5 2 From the information given, we know
that 4 is the object and 5 is the image.
Compare 1 and 2 , The both function is compared because
they are under the same function or in
Hence, other way there are having the same
4p + 3 = 5 object which is 4. So the value of p can be
4p = 2 calculated.

1
p=
2

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Example 2:

2x
The function g is defined by g ( x ) = If g(5) = 3g(2), find the value of m. Hence, find:
x+m

(a) image of 4.
(b) the value of x such that the function g is undefined.

2(5) 2( 2)
Answer: g (5) = and g (2) = . Replace
5+m 5+m
g(5) = 3g(2) 2(5) 2( 2)
g(5) with and g(2) with .
5+ m 5+m
2( 5)
5+ m
= 3. 2+2m
2( )

60 + 12m = 20 + 10m
2m = - 40 Any number that is divided by zero will result
m = -20 undefined or infinity. The value of denominator
2( 4) cannot equal to zero because it would cause
(a) g (4) = the solution becomes undefined or infinity. To
4 − 20
1 find the value of x to make the function
= − undefined, the denominator must be equal to
2
zero.
(b) x − 20 = 0
x = 20

EXERCISE 1.2
1. Given function f is defined by f ( x ) = 5 x − 4 . Find the value of x if f ( x ) = −14 .
2. Given that f ( x) = 2 x − 3 . Find the image of 5.
2x − 3
3. Given that function f such that f ( x ) = .
x−4
(i) The image of 5.
(ii ) the value of x such that the function f is undefined
4. Given the function h such that h( x) = 2 x 2 + 3kx + 1 and h(1) = 9 . Find the value of k.
5. Given the function f : x → 3x + 1 , find
(a) the image of 2,
(b) the value of x if f(x) = x.
6. Given the function w : x → 4 x + 3 , find the value of p such that w (p) = 23 .
7. Given the function n : x → 2 x − 5 . Find the value of k at which n(k ) = k .

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1.3 COMPOSITE FUNCTIONS
The figure shows Set A, B and C.
2
A f:x 3x B g:x x C The combined effect of two
functions can be represented by
the function gf(x) and not fg(x).
x 3x (3x)2
This is because the range of f has
1 3 9
become the domain of function g
2 6 36
or f(x) becomes the domain of
function g.

The concept is:
f(x) = 3x
gf(x) = g[f(x)] gf
f(k) = 3(k)
= g(3x)
f(3) = 3(3)
= (3x)2
This is f(x) which has become the object of f[g(x) ]= 3g(x)
function g

Example 1:
Given function f : x → x + 1 and g : x → 3x . Find the composite function gf.

f(x) = x + 1
g(x) = 3x
Replace f(x) with its image which is (x +1)
gf(x) = g[f(x)]
= g(x +1)
= 3(x +1) (x +1) has become the object of function g which is
replacing x. [g(x)=3x and g(x+1)=3(x+1)]. When x in the
Hence, gf: x 3(x +1) object is replaced by (x+1) so x in the image is also
replaced by (x+1) and become 3(x+1). We can also
expand the expression and becomes 3x + 3.

Example 2:
The following information refers to the functions f and g.

f : x → 2x − 3
4
g:x→ ,x ≠ k
x −1
Find
(a) the value of k,
(b) fg(x).

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Solution:
In Form One we have learned that any number that divided by zero will result infinity or undefined.
For part (a), if there is given an expression which is a fraction there must be given an information that
h
the value of unknown in the denominator that it cannot be. For example, g : x → , x ≠ 2 . The
x−2
part of denominator is x − 2 . If x = 2 , the denominator would be 0 and the solution will be undefined
or infinity. So x cannot equal to 2. For the question above, we know that x cannot equal to 1 because if
x = 1 ,the denominator would be 0 and the solution will be undefined or infinity. It is given that x ≠ k .
We already know that x ≠ 1 . So compare these two equations and it would result k = 1 . So 1 is the
answer for part (a)

For part (b),
4
f(x)=2x – 3 and) g ( x) = ,x ≠1
x −1

fg(x) = f[g(x)] 4
Replace g(x) with
4 x − 1
= f( )
x −1
4
= 2( )- 3
x −1 f(x)=2x – 3 ,
8 4 4
= -3 f( )= 2( )- 3
x −1 x −1 x −1
8 3( x − 1)
= −
x −1 x −1
8 − 3( x − 1)
=
x −1
8 − 3x + 3
=
x −1
11 − 3x
= , x ≠1
x −1

Example 3:
The following information refers to the functions f and g.

f : x → 4 − 3x
3
g:x→ ,x ≠ 2
x−2

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Find
(a) the value of gf(1)
(b) f2

Solution:
For part (a), at first we have to find gf(x).
3
Given f ( x) = 4 − 3x and g ( x ) = ,x ≠ 2
x−2
Replace f(x) with 4 − 3 x
gf(x)= g[f(x)]
= g(4 – 3x)
3
=
( 4 − 3 x) − 2
3
3 g( x ) =
= x−2
2 − 3x
3
3 2 g(4 x − 3) =
gf(x)= ,x ≠ ( 4 − 3x ) − 2
2 − 3x 3
3
gf(1)=
2 − 3(1)
3
=
2−3
3
=
−1
= −3

For part (b), f2 means ff.
Replace f(x) with 4 − 3 x
Given f ( x) = 4 − 3x

ff(x)= f[f(x)]
= f [ 4 − 3 x] f ( x) = 4 − 3 x ,
= 4 − 3(4 − 3x ) f [4 − 3 x] = 4 − 3(4 − 3x )
= 4 − 12 + 9 x
= 9x − 8
f 2 ( x) = 9x − 8

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1.3.1 Determining one of the function in a given composite function

(1)The following information refers to the functions f and fg.

f : x → 2x − 3
11 − 3 x
fg : x → ,x ≠1
x −1 Remember the concept…
f(x) = 2x – 3
Find function g. f(k) = 2k – 3
f(3) = 2(3) – 3
Solution: f[g(x)]= 2g(x) – 3
11 − 3 x
Given f ( x) = 2 x − 3 and fg ( x ) = ,x ≠1
x −1
If f ( x) = 2 x − 3 ,
If f ( y ) = 2 y − 3
So f [ g ( x )] = 2[ g ( x )] − 3
fg ( x ) = 2[g( x )] − 3
11 − 3 x
Given fg ( x ) = , x ≠ 1.
x −1
Compare the two equations.
Hence,
11 − 3 x
2[ g ( x)] − 3 =
x −1
11 − 3x
2[ g ( x)] = +3
x −1
11 − 3x 3( x − 1)
2[ g ( x)] = +
x −1 x −1

11 − 3x + 3( x − 1)
2[ g ( x)] =
x −1

11 − 3x + 3 x − 3
2[ g ( x)] =
x −1

8
2[ g ( x )] =
x −1
Simplify 8 and 2
8
[ g ( x )] =
2( x − 1)

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4
g ( x) =
x −1

4
Hence g : x → ,x ≠1
x −1

(2) The following information refers to the functions f and fg.

4 4
g:x→ g:x→
x −1 x −1
11 − 3x 11 − 3x
fg : x → , x ≠ 1 fg : x → ,x ≠1
x −1 x −1

Find function f.

Solution:
11 − 3 x
f [ g ( x )] =
x −1
 4  11 − 3x
f = 1
 x −1 x −1
Let
4
=y 2
x −1
4 = xy − y
xy = 4 + y
4+ y
x= 3
y
Substitute 2 and 3 into 1 ,
4+ y
11 − 3( )
y
f ( y) =
4+ y
−1
y

12 + 3 y
11 − ( )
y
= Convert 1 to a fraction
4+ y y
−
y y

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11 y 12 − 3 y
−
y y
=
4+ y− y
y

11 y − 12 − 3 y
y
=
4
y

8 y − 12 8 y − 12 4
It means divided by
y y y
=
4
y
Simplify
8y − 12 y
= ×
y 4

= 2y − 3
f ( y) = 2 y − 3
f ( x) = 2 x − 3

EXERCISE 1.3
x +1
1. Function f and gf are given as f ( x ) = and gf ( x) = 3 x + 2 respectively. Find function g.
2
2. Given f ( x) = 5 − 3x and g ( x ) = 4 x . Find function fg.
3. Given gf ( x) = x − 6 and g ( x ) = 4 x − 3 . Find
(i) function f
(ii) function fg
4. Given the function w ( x ) = 3 − 2 x and v ( x ) = x 2 − 1 , find the functions
(a) wv
(b) vw
2x − 1
5. Given the function w : x → x + 2 and v ( x ) → ,x ≠ k,
x −1
(a) state the value of k
(b) find
(i) wv(x)
(ii) vw(x)

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1.4 INVERSE FUNCTIONS
−1
If f ( x ) = y then f ( y) = x
Tips…
A B
f f −1 ≠ 1
f

X Y

−1
f

Inverse function

Example 1 x 3x + 1

Given f : x → 3x + 1 f
If x=1,
f (1) = 3(1) + 1
1 −1 4
= 3 +1 f
= 4
Hence,
f (1) = 4 x 3x +1
y −1
So, y
3
x −1
f-1(4) = 1 x
3

let
3x + 1 = y
3x = y − 1
y −1
x=
3
y −1
f-1(y) =
3
x −1
f-1(x) =
3
4 −1
f-1(4) =
3
= 1

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1.4.1 Determining the inverse function

Example 1:

5
Given g : x → , x ≠ 2 . Determine the inverse function g −1 .
x−2
Solution:
let
5
=y
x−2
xy − 2 y = 5
xy = 5 + 2 y
5 + 2y
x=
y
5 + 2y
g −1 ( y ) =
y
5 + 2x
g −1 ( x ) = ,x ≠ 0
x

Example 2:

The function f is defined by f : x → 3 − 4 x . Find f-1(-5)

1 Method 1

Let
3 − 4x = y
4x = 3 − y
3− y
x=
4
3− y
f −1 ( y ) =
4
3− x
f −1 ( x ) =
4
3 − (−5)
f −1 (−5) =
4
8
=
4

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−1
f (−5) = 2 image

2 Method 2
y -5
We know that f ( x ) = 3 − 4 x
And f −1 ( −5) = y

x 3 - 4x
image

Hence,
3 − 4 x = −5
4x = 3 + 5
4x = 8
x=2
So,
−1
f (−5) = 2

EXERCISE 1.4
1. Given f ( x) = 5 − 3x and g ( x ) = 4 x . Find
−1
(i) the value of f (−4)
(ii) the value of (gf ) −1 (−4)

−1
2. Given g ( x) = 5 − 3x and gf ( x) = 4 x . Find function f.
3. Given gf ( x) = x − 6 and g ( x ) = 4 x − 3 . Find
(i) (gf ) −1
(ii ) the value of fg (2)
2x
4. Given f : x → , x ≠1,
x −1
(a) find the function f −1
(b) state the value of x such that f −1 does not exist.
x
5. Show that the function f : x → , x ≠ 1 , has an inverse function that maps onto its self.
x −1
6. If h : x → x − 2 , find the value of p given that h −1 (p) = 8 .

Page | 13


1.5 ABSOLUTE FUNCTION
Absolute function means the value of image is always positive because of the modulus sign.

Given g : x → 2 − x and y= g ( x) .Find the value of y if x = 3

y= g ( x)
= | g (3) | Replace g(x) with is 2 − x
= | 2−3|
= | −1 | This is called modulus. Any value in the
=1 modulus would be positive. If the number
is positive, it remains positive but it is
negative it would change to positive
when remove the modulus sign.

1.5.1 Graph of absolute function

Example:
Given f ( x ) = 2 − x and y =| f ( x) | . Sketch the graph of y =| f ( x) | for the domain 0≤ x ≤ 6. Hence
state the corresponding range of y.

y =| f ( x) | Replace f(x) with its image which is 2 − x

=| 2 − x |

Minimum value of y=0 Minimum value of y must be zero
When 2-x = 0 (2, 0) because of absolute value cannot be
X=2 negative so the minimum it could be is
i) x = 0 zero.
y =| 2 − 0 |
=| 2 | (0, 2)
=2

ii) ) x = 6
y =| 2 − 6 |
=| −4 | (6 , 4)
=4

Then, sketch the graph using the points.

Page | 14


Range of y

Y

5
4
3 y =| f ( x) | Range of x

2
1
0 x
1 2 3 4 5 6

Hence, the corresponding range of y is 0≤ y ≤ 4

:: Some important information to be understood ::

1. Concept Of Solving Quadratic Equation by factorization
To make a product equal to zero, one of
x 2 − 3x − 4 = 0 them or both must be equal to zero. We
( x + 1)( x − 4) = 0 do not know either x-1 is equal to zero or
x + 1 = 0 or x − 4 = 0 x-4 equal to zero so that is why we use
x = −1 or x = 4 the word ‘or’ not ‘and.’

Example:
Given f ( x ) = 2 x 2 − 5 x . Find the possible values of k such that f −1
(3) = k
x 2x2 – 5x
Solution:
−1
f (3) = k
object k 3
image

Hence,
2 x 2 − 5x = 3 The general form of quadratic equation is
2 x 2 − 5x − 3 = 0 ax 2 + bx + c = 0 . To factorize and use the
(2 x + 1)( x − 3) = 0 concept, the equation must be equal to zero.

Page | 15


2 x + 1 = 0 or x − 3 = 0
1
x=− or x = 3
2
1 To make a product equal to zero, one of
f −1 (3) = − or f −1 (3) = 3
2 them or both must be equal to zero. We
Given that f −1 (3) = k do not know either 2x+1 is equal to zero
1 or x-3 equal to zero so that is why we use
k=− or k = 3 the word ‘or’ not ‘and.’
2

2. Concept of Square root and any root which is multiple of 2

In lower secondary we have learned that if x2 = 4 then x = 2. Actually the right answer for it is not 2 but
± 2 ( ± 2 is +2 and -2). This is because if (2)2, the answer is 4 and if (-2)2, the answer is also 4. So the
square root of 4 is ± 2 . It is also the same for fourth root, sixth root and so on. But in certain situation,
the answer will be only one.

Example:
Solve the equation 4 x 2 = 64 .

Solution:
4 x 2 = 64
x 2 = 16
x = ±4

3. Comparison method

Example:

h 12 + kx
Given the functions g : x → , x ≠ 2 and g −1 : x → , x ≠ 0 where h and k are constants,
x−2 x
find the value of h and of k.

Solution:

From the information given, we know that the method that we have to use to solve the question is
comparison method.

Page | 16


First, we have to find the inverse function of g.
Let
h
=y
x−2
yx − 2 y = h
yx = h + 2 y

h + 2y
x=
y

h + 2y
g −1 : y →
y
h + 2x
g −1 : x → ,x ≠ 0 1
x
Given that
12 + kx
g −1 : x → ,x ≠ 0 2
x
Both of them are inverse function of g.

So compare 1 and 2 ,
(h) + (2) x
g −1 : x → ,x ≠ 0
( x)
(12) + (k ) x
g −1 : x → ,x ≠ 0
( x)
Hence ,
h = 12 and k = 2

Page | 17


CHAPTER REVIEW EXERCISE
1. Diagram 1 shows the relation between set J and set K.

•2
•4
1•
•6
2•
•8
Set J
•10
Set K

DIAGRAM 1
State
(a) the codomain of the relation,
(b) the type of the relation.

2. Diagram 2 shows the graph of the function y = f(x).

y

y= f(x)
23
11

O x
1 3
It is given that f(x)= ax + b. DIAGRAM 2
Find
(a) the value of a and of b,
(b) the value of m if m is mapped onto itself under the function f,
(c) the value of x if f -1(x) = f 2(x).

kx − 4
3. Function h is given as h( x) = , x ≠ k . Find the value of k.
x −1
x+k
4. Given function g ( x ) = hx + 5 and g −1 ( x) = , find the values of h and k.
2
3
5. Given g : x → , x ≠ 2 . Find the values of x if g ( x ) = x .
x−2

Page | 18


k
6. Functions f and g are defined by f : x → 3x − h and g : x → , x ≠ 0 , where h and k are
x
−1
constants. It is given that f (10) = 4 and fg(2) = 16. Find the value of h and of k.

7. Given that f ( x) = 3 x − 2 and fg ( x ) = x 2 − 1 , find
(i) the function g
(ii) the value of gf(3)

8 Given f ( x ) = 2 x + 2 and y =| f ( x) | . Sketch the graph of y =| f ( x) | for the domain 0≤ x ≤ 4. Hence
state the corresponding range of y.

9. Given that function h is defined as h : x → px + 12 . Given that the value of 3 maps onto itself under
the function h, find
(a) the value of p
(b) the value of h −1 h(−2)

10. Given that the functions h( x) = 2 x − 1 and hh( x) = px + q , where p and q are constants, calculate
(a) the value of p and q
(b) the value of m for which h(−1) = 2m

11. Function g and composite function fg are defined by g : x → x + 3 and fg : x → x 2 + 6 x + 7 .
(a) Sketch the graph of y = g ( x) for the domain −5≤ x ≤ 3.
(b) Find f(x).

12. Given quadratic function g ( x ) = 2 x 2 − 3 x + 2 . find
(a) g(3),
(b) the possible values of x if g( x ) = 22
13. Functions f and g are given as f : x → x 2 and g : x → px + q , where p and q are constants.
(a) Given that f (1) = g(1) and f (3) = g (5) , find the value of p and of q.
(b) By using the values of p and q obtained in (a), find the functions
(i) gg
(ii) g −1
14. If w ( x ) = 3x − 6 and v ( x ) = 6 x − 1 , find
(a) wv (x )
(b) the value of x such that wv (−2 x ) = x .
15. Given f ( x ) = 3x − 6 . Find the values of x if f ( x ) = 3 .

Page | 19

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