Mathematics 9 Quadratic Functions (Module 1)

(Effective Alternative Secondary Education)
MATHEMATICS IV
Module 1
Quadratic Functions
1
BUREAU OF SECONDARY EDUCATION
Department of Education
DepEd Complex, Meralco Avenue,Pasig City

Module 1
Quadratic Functions
What this module is about
This module is about identifying quadratic functions, rewriting quadratic functions in
general form and standard form, and the properties of its graph. As you go over the
discussion and exercises, you will understand more about this function, and how to
differentiate it from a linear function which you have learned in the previous modules. Enjoy
learning and do not hesitate to go back if you think you are at a loss.
What you are expected to learn
This module is designed for you to:
1. identify quadratic functions f(x) = ax2
+ bx + c.
2. rewrite a quadratic function f(x) = ax2
+ bx + c in the form f(x) = a(x – h)2
+ k and vice
versa.
3. given a quadratic function, determine the
• highest or lowest point (vertex),
• axis of symmetry, and
• direction of opening of the graph.
How much do you know
1. Tell whether the function is quadratic or linear.
a. y = 2x + 3
b. y = 3x2
+ 5x – 6
c. f(x) = -2(x + 1)2
- 5
d. f(x) = 7(4x + 5)
e. f(x) = 9 – 2x
2. Which of the following table of ordered pairs represents a quadratic function?
a.
2
x -2 -1 0 1 2
y 1 -2 -3 -2 1

b.
c.
d.
3. What is f(x) = (x + 1)2
– 3 in general form?
a. f(x) = x2
+ 2x – 4
b. f(x) = x2
+ 2x – 2
c. f(x) = x2
+ 2x + 2
d. f(x) = x2
+ 2x + 4
4. How is f(x) = x2
– 6x + 14 written in standard/vertex form?
a. f(x) = (x + 3)2
+ 2
b. f(x) = (x – 3)2
– 5
c. f(x) = (x – 6)2
+ 5
d. f(x) = (x – 3)2
+ 5
5. Which of the following is the graph of a quadratic function?
a.
b.
c.
d.
6. What do you call the graph of a quadratic function?
a. Parabola
b. Line
c. Circle
d. Curve
3
x 0 1 2 3 4
y 2 5 8 11 14x -2 -1 0 1 2
y 8 1 0 -1 -8
x -1 0 1 2 3
y 10 7 4 1 -2
x
y
x
y
x
y
x
y

7. Which of the following quadratic functions will open upward?
a. y = 2 – 3x – 5x2
b. y = -(x + 4)2
c. y = -(3x2
+ 5x – 1)
d. y = 4x2
– 12x + 9
8. Determine the vertex of the quadratic function f(x) = 3x2
– 6x + 5.
a. (1, 2)
b. (-1, 2)
c. (1, -2)
d. (-1, -2)
9. Which of the following is the axis of symmetry of y = -2x2
+ 12x – 23?
a. x = -3
b. x = -5
c. x = 3
d. x = 5
10.What is the maximum value of f(x) = -5(x + 1)2
+ 4?
a. y = -5
b. y = 4
c. y = -1
d. y = -4
What you will do
Lesson 1
Identifying Quadratic Functions
Consider a rectangle whose width is (x + 2) cm and whose length is (x +
5) cm. How do you find the area of this rectangle?
The situation above is illustrated in the figure that follows.
4
(x + 5) cm
(x+2)cm

You recall that the formula for the area of a rectangle is A = lw where l is
the length and w is the width. Thus, the area of the given rectangle is –
A = [(x + 5) cm] [(x + 2) cm]
A = (x + 5)(x + 2) cm2
Multiplying the binomial then simplifying, the area of the rectangle is -
A = (x2
+ 5x + 2x + 10) cm2
A = (x2
+ 7x + 10) cm2
If x = 3, the area of the rectangle can be obtained by substituting in the
equation above.
A = 32
+ 7(3) + 10 cm2
A = 9 + 21 + 10 cm2
A = 40 cm2
Notice that in the given example, the area of the rectangle is a function of
its dimension. Thus, the area of the rectangle can also be written in functional
notation as
Area = f(x) = (x2
+ 7x + 10) cm2
.
Observe that the highest exponent is 2. Hence, the degree of f(x) = x2
+
7x + 10 is 2 which is called a quadratic function.
The following are examples of quadratic functions.
1. f(x) = x2
+ 7
2. f(x) = 6x2
– 4x + 3
3. y = 9 + 2x – x2
4. y = x2
– 6x - 16
5. f(t) = 




 −
2
1t
t
6. y = (x + 7)2
- 9
Why do you think the examples above are called quadratic functions?
The following are not quadratic functions.
1. y = 5x + 8
2. f(x) = 5x
+ 2
3. f(x) = x3
– 27
4. y = 32xx x
+−
5. y = x(x2
+ 7x - 1)
Why do you think the examples above are not quadratic functions?
5

Now that you know how to identify a quadratic function given an equation,
how will you identify a quadratic function from a given set of ordered pairs or a
table of values?
Example 1:
Consider the ordered pairs of values for the quadratic function f(x) = x2
for
the integers -3 ≤ x ≤ 3.
{(-3, 9), (-2, 4), (-1, 1), (0, 0), (1, 1), (2, 4), (3, 9)}
The ordered pairs of values above can also be presented using a table of
ordered pairs as shown below.
.
x -3 -2 -1 0 1 2 3
y 9 4 1 0 1 4 9
Observe the characteristics of a quadratic function, when ordered pairs
are given.
1 1 1 1 1 1 Differences in x
x -3 -2 -1 0 1 2 3
Y = f(x) 9 4 1 0 1 4 9
-5 -3 -1 1 3 5 Differences in y
Notice that the differences in x are equal while the differences in y are not.
Let us call the differences in y obtained above as first differences in y.
Look what happens when second differences in y are obtained.
x -3 -2 -1 0 1 2 3
y = f(x) 9 4 1 0 1 4 9
-5 -3 -1 1 3 5 First differences in y
2 2 2 2 2 Second differences in y
Observe that the second differences in y are equal. Hence, for the
quadratic function, f(x) = x2
, equal differences in x produce equal second
differences in y. The method presented above is called the equal differences
method.
Example 2:
6

Consider the table of values for the quadratic function f(x) = 5 – 2x2
.
x -3 -1 1 3 5 7 9
y = f(x) -13 3 3 -13 -45 -93 -157
Solution:
Verify if equal differences in x will produce equal second differences in y.
x -3 -1 1 3 5 7 9
y = f(x) -13 3 3 -13 -45 -93 -157
16 0 -16 -32 -48 -64 First differences in y
-16 -16 -16 -16 -16 Second differences in
y
Observe that like in f(x) = x2
, the table of values for f(x) = 5 – 2x2
showed
that equal differences in x produced equal second differences in y = f(x). This
is true for all quadratic functions.
Example 3:
Determine if the ordered pairs of numbers given in the table below
represents a quadratic function or not.
x -6 -4 -2 0 2 4 6 8
y -7 -3 1 5 9 1
3
1
7
21
Solution:
Apply the same method as seen in examples 1 and 2 and see if equal
differences in x will also produce equal second differences in y.
2 2 2 2 2 2 2
x -6 -4 -2 0 2 4 6 8
y -7 -3 1 5 9 1
3
1
7
21
4 4 4 4 4 4 4
7

Notice that the differences in x produced equal first differences in y.
Thus, the ordered pairs of numbers in the given table does not represent a
quadratic function.
Try this out
A. Tell whether the following functions are quadratic functions or not. Explain.
1. f(x) = x2
- 9
2. f(x) = 3x + 15
3. f(x) = 24 + 5x - x2
4. f(x) = 27 – 4x2
5. f(x) = 2(x-6)2
+ 1
6. f(x) = (3x + 2)(x - 5)2
7. f(x) = 5x2
+ x – 2
8. f(x) = 5
x-4
9. f(x) = x2
6
5
−
10.f(x)= 4
3
x
2
7
+
B. Using the equal differences method, determine which of the following
ordered pairs represent a quadratic function. Justify your answer.
1. {(-1, 11), (0, 6), (1, 3), (2, 2), (3, 3), (4, 6), (5, 11)}
2. {(-3, -35), (-2, -16), (-1, -9), (0, 0), (1, -7), (2, 0), (3,
19)
3. {(1, 5), (3, 13), (5, 29), (7, 53), (9, 85)}
4. {(-2, -13), (-1, -6), (0, -5), (1, -4), (2, 3)}
5. {(-5, 40), (-4, 28), (-3, 18), (-2, 10), (-1, 4), (0, 0)}
6.
7.
8.
8.
8.
x -5 -2 1 4 7
y 64 91 100 91 64
x -3 -2 -1 0 1
y 56 37 30 29 28
x -4 -2 0 2 4
y 39 24 9 -6 -21
8

9.
9.
9.
10.
10.
10.
C. The sum of two numbers is 12.
1. If one number is represented by x, what is the other
number?
2. If f(x) represents their product, express f(x) in terms of
x.
3. Give a table of values for this relation for the integers
0 ≤ x ≤ 12.
4. Determine whether the table of values represents a
quadratic function or not.
5. Determine the product of each pair of numbers.
6. Which pair of numbers in (5) gives the greatest
product?
7. Which pair of numbers in (5) gives the least product?
Lesson 2
Rewriting Quadratic Functions from f(x) = ax2
+ bx + c to f(x) =
a(x-h)2
+ k and vice versa
In lesson 1, you learned that f(x) = ax2
+ bx + c is the standard form of a
quadratic function. This function can be written in an equivalent form using the
process of completing the square. Study the steps as shown below.
f(x) = ax2
+ bx + c Standard form of a quadratic function
f(x) = cx)
a
b
a(x2
++ Factor out a from x2
and x terms
x -3 -1 1 3 5
y 25 10 -5 -20 -35
x -10 -5 0 5 10
y -20 -85 -100 -85 -20
9
Complete the square by adding and
subtracting
2
2a
b
a 






f(x) =
22
2
2a
b
ac
2a
b
x
a
b
xa 





−+














++
f(x) = 





−+





++ 2
2
2
2
2
4a
b
ac
4a
b
x
a
b
xa
f(x) =
4a
b
c
4a
b
x
a
b
xa
2
2
2
2
2
−+





++ Simplify 





2
2
4a
b
a to
4a
b2
f(x) =
4a
b4ac
2a
b
xa
22
−
+





+
From the result let k
4a
b4ac
andh
2a
b 2
=
−
−= . Substituting this to the equation
above will result to f(x) = a(x – h)2
+ k. Hence, f(x) = ax2
+ bx + c is equivalent to
f(x) = a(x – h)2
+ k.
Examples:
Rewrite the following quadratic functions in the form f(x) = a(x- h)2
+ k.
1. f(x) = x2
- 2x - 15
2. f(x) = 2x2
- 7
3. y = 4 + x – 3x2
4. y = 4x2
+ 5x
Solution 1: Using completing the square.
1. f(x) = x2
- 2x – 15
f(x) = (x2
- 2x) – 15 Factor out 15 in the x terms.
f(x) = (x2
- 2x + 1) - 15 – 1
f(x) = (x - 1)2
– 16
2. f(x) = 2x2
– 7
f(x) = 2x2
- 0x – 7
f(x) = 7x
2
0
x2 2
−





− Factor out 2 in the x terms.
f(x) = 2 070x
2
0
x2
−−





+−
f(x) = 2(x - 0)2
– 7
10
Factor the trinomial inside the
parenthesis and simplify the last 2
terms
Complete the square inside the
parenthesis by adding and
subtracting 1.
Factor the trinomial inside the bracket
and simplify the last two terms
Expand the terms added and subtracted
in the previous step
parenthesis and combine the last
two terms.
subtracting 0.
Write function in the form f(x) = ax2
+ bx + c.

3. y = 4 + x - 3x2
y = -3x2
+ x + 4
y = -3 





− x
3
1
x2
+ 4 Factor out -3 in the x terms
y =
22
2
6
1
3)(4
6
1
x
3
1
x3 





−−−+














−+−−
y = 





−−+





+−−
36
1
)3(4
36
1
x
3
1
x3 2
Square the added number.
y = 





−−+





−−
12
1
4
6
1
x3
2
f(x) = 12
49
6
1
x3
2
+





−− Add the last two terms
4. y = 4x2
+ 5x Given
y = 4 





+ x
4
5
x2
Factor out 4 in the x terms
y = 4
22
2
8
5
4
8
5
x
4
5
x 





−














++
y = 4 





−





++
64
25
4
64
25
x
4
5
x2
Square the added number
y = 4
16
25
8
5
x
2
−





+
Solution 2:
Using the formula in solving the values of h and k:
In the relation –h = 2a
b
, the value of h can be obtained using the
multiplication property of equality so that
a2
b
h
−
= .
1. f(x) = x2
- 2x - 15.
Substitute the values a = 1, b = -2, and c = -15 in the formula.
1
2
2
2(1)
2)(
2a
b
h ==
−−
=
−
=
16
4
64
4
460
4(1)
2)(15)4(1)(
4a
b4ac
k
22
−=
−
=
−−
=
−−−
=
−
=
11
parenthesis and reduce the fraction
to the lowest term.
subtracting
2
6
1
3 





−−
Rewrite the equation in the
form y = ax2
+ bx + c.
parenthesis and reduce the fraction
to the lowest term
subtracting
2
8
5
4 





−

Substituting the values of h and k to f(x) = a(x – h)2
+ k.
Thus, f(x) = x2
– 2x – 15 is equivalent to f(x) = (x – 1)2
- 16.
2. f(x) = 2x2
– 7.
Substitute the values a = 2, b = 0 and c = -7 in the formula.
h = 0
4
0
)2(2
0
a2
b
==
−
=
−
k = 7
8
56
8
056
)2(4
0)7)(2(4
a4
bac4 22
−=
−
=
−−
=
−−
=
−
Substitute the values of h and k to f(x) = a(x – h)2
+ k.
Therefore, f(x) = 2x2
– 7 is equivalent to f(x) = 2(x – 0)2
– 7.
3. y = 4 + x – 3x2
. Here, a = -3, b= 1, c = 4
6
1
6-
1
3)2(
(1)
2a
b
h =
−
=
−
−
=
−
=
12
49
12
49
12
148
3)4(
(1)3)(4)4(
k
2
=
−
−
=
−
−−
=
−
−−
=
Thus, y = 12
49
6
1
-x3
2
+





− .
4. y = 4x2
+ 5x. Here a = 4, b = 5 and c = 0.
8
5-
2(4)
5-
2a
b-
h ===
( )
16
25-
16
25-0
)4(4
5-4(4)(0)
k
2
===
Hence, 





+











=
16
25
-
8
5
--x4y
2
or
16
25
8
5
x4y
2
−





+=
Observe that the two solutions resulted to the same answer. Thus, a
quadratic function in the form f(x) = ax2
+ bx + c can be transformed in the form
f(x) = a(x – h)2
+ k by completing the square or the relation h =
a2
b−
and
a4
bac4
k
2
−
= .
12

Now, how will you transform a quadratic function in the form f(x) = a(x –
h)2
+ k to the standard form f(x) = ax2
+ bx + c?
To do this, simply follow the given steps.
1. Expand the square of the binomial indicated in the function.
2. Multiply the result by the value of a.
3. Combine the similar terms.
Now, study the examples below.
Examples:
Transform the following equation to standard form.
1. f(x) = (x – 3)2
– 7
2. f(x) = -2[x – (-5)]2
+ 50
3. y = 5(x + 4)2
– 3
4. y =
4
3
− ( ) 92
2
+−x
Solutions:
1. f(x) = (x – 3)2
+ 7
f(x) = x2
– 6x + 9 + 7 Square the binomial
f(x) = x2
– 6x + 16 Combine the similar terms
2. f(x) = -2[x – (-5)]2
+ 50
f(x) = -2(x + 5)2
+ 50 Simplify the term inside the parenthesis
f(x) = -2(x2
+ 10x + 25) + 50 Square the binomial
f(x) = -2x2
- 20x -50 + 50 Multiply the result by -2
f(x) = -2x2
- 20x Combine the similar terms
3. y = 5(x + 4)2
– 3
y = 5(x2
+ 8x + 16) – 3 Square the binomial
y = 5x2
+ 40x + 80 – 3 Multiply the result by 5
y = 5x2
+ 40x + 77 Combine the similar terms
4. y =
4
3
− ( ) 92
2
+−x
y =
4
3
− ( ) 9442
++− xx Square the binomial
y =
4
3
− 9332
+−+ xx Multiply the result by
4
3
−
y =
4
3
− 632
++ xx Combine the similar terms
13

Try this out
A. Rewrite the following quadratic functions to f(x) = a(x – h)2
+k.
1. f(x) = 2x2
- 12x + 33
2. f(x) = x2
+ 8x
3. f(x) = 5x2
- 6
4. f(x) = 3x2
+ 24x + 43
5. f(x) = 0.5x2
+ 11
6. f(x) = 1 + 16x - 8x2
7. f(x) = 12 – x2
8. f(x) = 15x
2
3 2
+
9. f(x) = x7
3
5 2
−
10.f(x) =
2
75xx4 2
+−
B. Transform the following quadratic function to f(x) = ax2
+ bx + c.
1. (x) = 3(x – 2)2
+ 5
2. f(x) = -7(x + 1)2
– 3
3. f(x) = 5(x - 9)2
+ 1
4. f(x) = -5(x - 3)2
+ 2
5. f(x) = (x + 5)2
– 12
6. f(x) = -4(x -1)2
+ 14
7. f(x) = 4(x -
2
1
)2
- 2
8. f(x) =
3
2 2
5)(x −
9. f(x) =
7
4
− (x – 1)2
+ 3
10.f(x) =
7
6
7)-(x 2
- 1
Lesson 3
Properties of the Graph of a Quadratic Function
The graph of a quadratic function is called a parabola. It is the set of all
points on the Cartesian Coordinate Plane that satisfies the function defined by
f(x) = ax2
+ bx + c or the vertex form f(x) = a(x – h)2
+ k where (h, k) is the vertex.
.
Look at the two graphs. What do you notice about the parabolas? In what
way are they similar? In what way are they different?
14-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
-4 -3 -2 -1 0 1 2 3 4
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
-4 -3 -2 -1 0 1 2 3 4
f(x) = x2
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
-4 -3 -2 -1 0 1 2 3 4
y = -x2
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
-4 -3 -2 -1 0 1 2 3 4
Y Y
X

The following properties of the parabolas should be observed.
1. The graph of y = x2
opens upward while the graph of y = -x2
opens
downward.
The direction of opening is indicated by the sign of a in the equation. Note
that in y = x2
, a is positive or a > 0 while in y = -x2
, a is negative or a < 0.
2. The two graphs have turning points. The turning point is called a vertex.
The vertex maybe the minimum point or the maximum point of the
parabola depending on the direction of opening of the graph.
The vertex of y = x2
is at (0, 0) also denoted by V(0, 0). It is the lowest or
minimum point on the graph. It is the minimum point if the parabola opens
upward
The vertex of y = -x2
is also at V(0,0) but it is the highest or maximum point
on the graph. It is the maximum point if the parabola opens downward.
3. Drawing a vertical line through the vertices of each graph divide both
graphs into two congruent or symmetrical parts such that one part is a
mirror image of the other. We call this line axis of symmetry. Thus, the
axis of symmetry of both graphs is the y-axis or the line x = 0.
15
y = -x2
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
-4 -3 -2 -1 0 1 2 3 4
y = x2
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
-4 -3 -2 -1 0 1 2 3 4
- 10
- 9
- 8
- 7
- 6
- 5
- 4
- 3
- 2
- 1
0
1
2
3
- 4 - 3 - 2 - 1 0 1 2 3 4
Vertex
Vertex
Axis of
Symmetry
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
-4 -3 -2 -1 0 1 2 3 4
Axis of
Symmetry
X
X
X
Y Y

-4
-3
-2
-1
0
1
2
3
4
5
6
-2 -1 0 1 2 3 4
Example 2:
Draw the graph of f(x) = 2(x - 1)2
– 3 and give the properties the function.
Solution:
Step 1: Construct a table of values for x and f(x). For this particular example, let
us use for x the values { -1, 0, 1, 2, 3}.
Substitute these values in f(x) = 2(x - 1)2
– 3.
f(-1) = 2(-1 - 1)2
- 3 = 2(-2)2
- 3 = 2(4) - 3 = 8 - 3 = 5
f(0) = 2(0 - 1)2
- 3 = 2(-1)2
- 3 = 2(1) - 3 = 2 - 3 = -1
f(1) = 2(1 - 1)2
- 3 = 2(0)2
- 3 = 2(0) - 3 = 0 - 3 = -3
f(2) = 2(2 - 1)2
- 3 = 2(1)2
- 3 = 2(1) - 3 = 2 - 3 = -1
f(3) = 2(3 - 1)2
- 3 = 2(2)2
- 3 = 2(4) - 3 = 8 - 3 = 5
The results in a table.
x -1 0 1 2 3
f(x) = 2(x – 1)2
- 3 5 -1 -3 -1 5
Step 2: Plot and connect the points on the Cartesian Plane.
The following properties of the quadratic function can
be observed from the parabola.
The function is f(x) = 2(x – 1)2
– 3 in the form
f(x) = a(x – h)2
+ k
Here, a = 2, h = 1 and k = -3.
1. The parabola opens upward because a > 0.
2. The vertex (h, k) is (1, -3), the lowest point of the
graph.
3. The axis of symmetry is x = 1, the value of h in
the vertex.
4. The minimum value is y = -3 which is the value of
k in the vertex.
16
(1, - 3)
X
Y

Hence, for any quadratic function, its vertex is at the point V(h, k) where
h = a2
b−
and
a4
bac4
k
2
−
= . You have learned this in the previous lesson.
Example 3:
Determine the direction of opening, vertex, axis of symmetry, and minimum
or maximum point of the quadratic function defined by y = -2x2
+12x – 5.
Solution: In the given equation, a = -2, b = 12, and c = -5.
a. Opening: downward since a is negative or a < 0.
b. Vertex:
Since the function is not in the vertex form f(x) = a(x – h)2
+ k, we cannot
easily determine the vertex (h, k).
Use the formula for h and k:
13
8
104-
8-
144-40
4(-2)
12-5)2)(4(
4a
b-4ac
k
3
4
12
2)2(
12-
2a
b
h
22
=
−
==
−−
==
=
−
−
=
−
=
−
=
Hence, the vertex is (3, 13);
c. The axis of symmetry is x = 3
d. The maximum value is y = 13 since the parabola opens downward.
Try this out
A. Give the sign of the leading coefficient, the coordinates of the vertex,
the axis of symmetry, and the highest/lowest value of the quadratic function
represented by the given parabolas.
1. 2.
17
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
-4 -3 -2 -1 0 1 2 3 4
-4
-3
-2
-1
0
1
2
3
4
5
6
7
-4 -3 -2 -1 0 1 2 3 4
Y Y

3. 4.
5. 6.
7. 8.
18
-5
-4
-3
-2
-1
0
1
2
-4 -3 -2 -1 0 1 2 3 4
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
-2 -1 0 1 2 3 4 5 6
-4
-3
-2
-1
0
1
2
3
4
5
6
-5 -4 -3 -2 -1 0 1 2 3 4 5
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
-2 -1 0 1 2 3 4 5 6
-6
-5
-4
-3
-2
-1
0
1
2
3
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
-4 -3 -2 -1 0 1 2 3 4
X
X
X
X
X
X
Y
Y
Y
Y
Y
Y

9. 110.
B. Determine the direction of opening of the parabola, the vertex, the axis of
symmetry and minimum or maximum value of the following quadratic
functions.
1. f(x) = (x - 3)2
2. f(x) = -(x +2)2
- 1
3. f(x) = - x2
+ 9
4. f(x) =
2
3
74)(x
2
+−
5. f(x) = -4(x - 6)2
+ 5
6. f(x) = x2
+ 3x – 4
7. f(x) = 2x2
+ 4x - 7
8. f(x) = x2
- x - 20
9. f(x) = 5 + 3x – x2
10. f(x) = 2 - 5x - 3x2
Let’s Summarize
19
-3
-2
-1
0
1
2
3
4
5
6
7
8
-6 -4 -2 0 2 4
-6
-5
-4
-3
-2
-1
0
1
2
3
4
-7 -6 -5 -4 -3 -2 -1 0 1 2 3
X
X
X
X
YY

1. A quadratic function is a second degree function in the form f(x) = ax2
+ bx + c where
a ≠ 0.
2. Quadratic functions can be written in two forms- the standard form f(x) = ax2
+ bx + c
or its equivalent form f(x) = a(x – h)2
+ k.
3. To rewrite a quadratic function from the form f(x) = ax2
+ bx + c to the form f(x) = a(x –
h)2
+ k, use completing the square; or determine the values of a, b, and c then solve
for h and k. Substitute the obtained values in f(x) = a(x – h)2
+ k. To find the values of
h and k, use the relationships,
4a
b-4ac
kand
2a
b
h
2
=
−
=
4. To rewrite a quadratic function from the form f(x) = a(x-h)2
+ k to the form f(x) = ax2
+
bx + c, expand the square of the binomial, multiply by a and add k, then simplify by
combining similar terms.
5. The graph of a quadratic function is called a parabola.
6. A parabola may open upward or downward depending upon the sign of a. If a>0, the
parabola opens upward while if a<0, the parabola opens downward.
7. The highest or lowest point of a parabola is called the turning point or vertex. It is
denoted by the ordered pair V(h, k) where
4a
b-4ac
kand
2a
b
h
2
=
−
=
x = h is called the axis of symmetry while y = k is the highest or lowest value of f(x).
8. The axis of symmetry is the line that passes through the vertex and divides the
parabola into two equal parts such that one part is the mirror image of the other.
What have you learned
1. Which of the following functions is quadratic?
a. y = x + 3
b. y = 5 – 6x2
c. f(x) = 2x + 1
- 5
d. f(x) = 7(4x + 5)
2. Which of the following table of ordered pairs represents a quadratic function?
a.
20
x -2 -1 0 1 2
y -8 -4 0 4 8

b.
c.
d.
3. What is f(x) = 2(x - 1)2
+ 8 in general form?
a. f(x) = 2x2
- 2x + 9
b. f(x) = 2x2
- x + 10
c. f(x) = 2x2
+ 4x + 8
d. f(x) = 2x2
- 4x + 10
4. How is f(x) = x2
- 10x + 29 written in standard/vertex form?
a. f(x) = (x - 5)2
+ 4
b. f(x) = (x + 5)2
+ 4
c. f(x) = (x – 5)2
- 4
d. f(x) = (x + 5)2
- 4
5. Which of the following is the graph of a quadratic function?
a.
b.
c.
d.
6. What do you call the highest or lowest point in the graph of a quadratic function?
a. Axis of symmetry
21
x 0 1 2 3 4
y -2 1 4 7 10
x -2 -1 0 1 2
y 9 2 1 0 -7
x -1 0 1 2 3
y 13 7 5 7 13
x
y
x
y
x
y
x
y

b. Slope
c. Vertex
d. Major axis
7. Which of the following quadratic functions will open downward?
a. f(x) = 2x2
– 3x – 5
b. f(x) = (x - 4)2
c. f(x) = 3 + 5x + 2x2
d. f(x) = 4 – 2x - 9x2
8. Determine the vertex of the quadratic function f(x) = 2x2
– 12x + 11.
a. (3, 7)
b. (-3, -7)
c. (3, -7)
d. (-3, -7)
9. Which of the following is the axis of symmetry of y = -3x2
- 30x – 75?
a. x = 5
b. x = -5
c. x = 3
d. x = -3
10.What is the minimum value of f(x) = 2(x + 1)2
- 7?
a. y = -7
b. y = 7
c. y = 1
d. y = 2
22

Answer Key
How much do you know
1. a. Linear Function d. Linear Function
b. Quadratic Function e. Linear Function
c. Quadratic Function
2. a
3. b
4. d
5. c
6. a
7. d
8. a
9. c
10.b
Lesson 1
A. 1. Quadratic function since the degree of the function is 2.
2. Not a quadratic function since the degree of the function is 1,
3. Quadratic function since the degree of the function is 2.
6. Not a quadratic function since the degree of the function is 3.
B. 1. Quadratic function; equal differences in x produced equal second
differences in y.
2. Not a quadratic function; equal differences in x did not produce equal second
differences in y.
3. Quadratic function; equal differences in x produced equal second differences in
y.
differences in y.
y.
y.
differences in y.
differences in y.
differences in y.
23

10.Quadratic function; equal differences in x produced equal second differences
in y.
C.
1. 12 – x
2. f(x) = x(12 – x) or f(x) = 12x – x2
3.
3.
3.
3.
3.
4. Quadratic Function, equal differences in x produced equal second
differences in y or f(x).
5.
6. (8, 32)
7. (0, 0) and (12, 0)
Lesson 2
A. 1. f(x) = 2(x – 3)2
+ 15
2. f(x) = (x + 4)2
– 16
3. f(x) = 5(x – 0)2
– 6 or f(x) = 5(x + 0)2
– 6
4. f(x) = 3(x + 4)2
– 5
5. f(x) = 0.5(x – 0)2
+ 11 or f(x) = 0.5(x + 0)2
+ 11
6. f(x) = -8(x – 1)2
+ 9
7. f(x) = -(x – 0)2
+ 12 or f(x) = -(x + 0)2
+ 12
8. f(x) = ( ) 150x
2
3 2
+− or f(x) = ( ) 150x
2
3 2
++
9. f(x) = - 7(x – 0)2
+
3
5
or f(x) = -7(x + 0)2
+
3
5
10. ( ) 32
872
8
5
2)( +−= xxf
B. 1. f(x) = 3x2
– 12x + 17
2. f(x) = -7x2
– 14x – 10
x 0 1 2 3 4 5 6 7 8 9 10 11 12
y or f(x) 0 1
1
1
0
2
7
32 35 36 35 32 27 20 11 0
product 0 1
1
4
0
8
1
128 175 216 245 256 243 200 121 0
x 0 1 2 3 4 5 6 7 8 9 1
0
1
1
12
y 0 1
1
2
0
2
7
3
2
3
5
3
6
3
5
3
2
2
7
2
0
1
1
0
24

3. f(x) = 5x2
– 90x + 406
4. f(x) = -5x2
+ 30x – 43
5. f(x) = x2
+ 10x + 13
6. f(x) = -4x2
+ 8x + 10
7. f(x) = 4x2
– 4x – 1
8. f(x) = )25x10x(
3
2 2
+− or f(x) =
3
50
x
3
20
x
3
2 2
+−
9. f(x) =
7
17
7
8
7
4 2
++− xx
10.f(x) = x12x
7
6 2
− + 41
Lesson 3
A.
B.
No. Direction of
Opening
Vertex Axis of
Symmetry
Minimum/Maximum Value
No
.
Sign of leading
coefficient
Vertex Axis of
Symmetry
Minimum/Maximum
value
1 Positive (0, 2) x = 0 or y-axis Minimum value; y = 2
2 Positive (0, -3) x = 0 or y -axis Minimum value; y = -3
3 Negative (0, 0) x = 0 or y-axis Maximum value; y = 0
4 Negative (0, 5) x = 0 or y-axis Maximum value; y = 5
5 Positive (2, -1) x = 2 Minimum value; y = -1
6 Negative (2, 3) x = 2 Maximum value; y = 3
7 Positive (-3, -5) x = -3 Minimum value; y = -5
8 Positive (0, -2) x = 0 or y-axis Minimum value; y = -2
9 Negative (-2, 3) x = -2 Maximum value, y = 3
10. Positive (-1, -2) x = -1 Minimum value, y = -2
25

1 Upward (3, 0) x = 3 Minimum value, y = 0
2 Downward (-2, -1) x = - 2 Maximum value, y = -1
3 Downward (0, 9) x = 0 or y-axis Maximum value, y = 9
4 Upward (4, 7) x= 4 Minimum value, y = 7
5 Downward (6, 5) x = 6 Maximum value, y = 5
6 Upward






−−
4
25
,
2
3
x = -
2
3
Minimum value, y = -
4
25
7 Upward (-1, -9) x = -1 Minimum value, y = -9
8 Upward






−
4
81
,
2
1 x =
2
1
Minimum value, y = -
4
81
9 Downward






4
29
,
2
3 x =
2
3
Maximum value, y =
4
29
10 Downward






−
12
49
,
6
5
x = -
6
5
Maximum value, y =
12
49
What have you learned
1. b 6. c
2. d 7. d
3. d 8. c
4. a 9. b
5. c 10.a
26

Mathematics 9 Quadratic Functions (Module 1)

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Mathematics 9 Quadratic Functions (Module 1)