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“Teach A Level Maths”
Vol. 1: AS Core Modules
3: Quadratic Expressions
Expanding Brackets and
Factorisation
© Christine Crisp
Quadratic Expressions
Module C1
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Quadratic Expressions
Expanding Quadratic Expressions
e.g. 1
( x  5)( x  2)
 x 2  2 x  5 x  10
 x 2  3 x  10
e.g. 2
( x  3)
2
A square of a quantity
means multiply by itself
 ( x  3)( x  3)
 x  3x  3x  9
2
 x  6x  9
2
Quadratic Expressions
Exercises
1.
( x  3)( x  4)
2.
( x  4)( x  5)
 x  4 x  3 x  12
2
 x  x  12
2
 x  x  20
3.
(2 x  1)( x  2)
 2x  5x  2
4.
( x  4)
2
2
2
 ( x  4)( x  4)
 x  8 x  16
2
Quadratic Expressions
Factorising
Method 1: Common Factors
e.g.
x  3 x means
x  x  3 x
2
so, x is a factor of both terms. It is a
common factor
So,
x  3 x  x ( x  3)
2
Common factor
Quadratic Expressions
Exercises
Factorise the following by taking out the
common factors
1.
x  5x
 x ( x  5)
2.
3x2  6x
 3 x ( x  2)
3.
2x  4x  8
 2( x  2 x  4)
2
2
2
Quadratic Expressions
 x ( x  4)
4.
x 2  4x
5.
x  3x
6.
4 x  12 y
7.
3x 2  6x  9
8.
3
2
 x ( x  3)
2
 4( x  3 y )
 3( x 2  2 x  3)
5 x (a  b)  2 y (a  b)  (a  b)(5 x  2 y )
Quadratic Expressions
Factorising
Method 2: The difference of two squares
(Square roots)
e.g.1
x 9
 ( x  3)( x  3)
2
A minus sign
One square
e.g.2
Another square
4 x 2  25 y 2  (2 x  5 y )( 2 x  5 y )
Quadratic Expressions
Exercises
1.
81  y 2
 (9  y )(9  y )
2.
4x2  9
 (2 x  3)( 2 x  3)
3.
3  12 x
 3(1  4 x )
2
2
Think!
4.
Common
factor first!
 3(1  2 x )(1  2 x )
9 x  16 y  (3 x  4 y )( 3 x  4 y )
2
2
What about
x2  4
Can’t do it!
?
It’s NOT a difference
Quadratic Expressions
Factorising
Method 3: Trinomials
e.g. 1
x  3x  4
2
22
or
41
 ( x  4 )( x  1 )
The factors 2  2 could4 not
x give 3 for the
coefficient of x, so we1 try
x 41
We need –3x so we want
–3x is
– 4x
and
+1x.
called the linear term
Quadratic Expressions
Method 3: Trinomials
e.g. 2
x  7x  6
2
61
23
 ( x  6)( x  1)
Constant positive

Signs of factors are the same
Quadratic Expressions
Exercises
Constant positive

Signs of factors are the same
1.
x  5x  6
 ( x  2 )( x  3 )
2.
x 2  5x  6
 ( x  2 )( x  3 )
3.
x 2  7 x  12  ( x  3 )( x  4 )
2
Quadratic Expressions
Exercises
Constant negative

Signs of factors are different
4.
x 2  5x  6
 ( x  1 )( x  6 )
5.
x 2  5x  6
 ( x  6 )( x  1 )
6.
x 2  2x  3
 ( x  1 )( x  3 )
Quadratic Expressions
Exercises
7.
x  5x  6
 ( x  2)( x  3)
8.
x  4 x  21
 ( x  7)( x  3)
9.
x  4 x  12
 ( x  2)( x  6)
10.
x 2  8 x  12
 ( x  6)( x  2)
11.
3 x 2  14 x  15
 (3 x  5)( x  3)
12.
4 x 2  16 x  15
 (2 x  5)( 2 x  3)
2
2
2
SUMMARY
Quadratic Expressions
There are 3 methods of factorising quadratic
expressions.
 Common factors.
 The difference of 2 squares.
 Trinomial factors.
• List possible pairs of factors of the constant.
• Constant term positive  signs are the same.
• Constant term negative  one sign is positive
and one is negative.
• Choose a pair of factors of the constant and
check the linear term is correct. If not, try
again.
Quadratic Expressions
The following slides contain repeats of
information on earlier slides, shown without
colour, so that they can be printed and
photocopied.
For most purposes the slides can be printed
as “Handouts” with up to 6 slides per sheet.
Quadratic Expressions
SUMMARY
There are 3 methods of factorising quadratics.
 Common factors.
 The difference of 2 squares.
 Trinomial factors.
• List possible pairs of factors of the constant.
• Constant term positive  signs are the same.
• Constant term negative  one sign is positive
and one is negative.
• Choose a pair of factors of the constant and
check the linear term is correct. If not, try
again.
Examples
Quadratic Expressions
x 2  5 x  x ( x  5)
2. 3 x 2  6 x  3 x ( x  2)
2
2
3. 2 x  4 x  8  2( x  2 x  4)
4. x 2  4 x  x ( x  4)
2
3
2

x
( x  3)
5. x  3x
6. 4 x  12 y  4( x  3 y )
1.
7. 3 x 2  6 x  9  3( x 2  2 x  3)
8. 5 x (a  b)  2 y (a  b)
 (a  b)(5 x  2 y )
81  y 2  (9  y )(9  y )
2
10. 4 x  9  (2 x  3)( 2 x  3)
9.
Quadratic Expressions
Examples
11. 3  12 x
2
 3(1  4 x 2 )  3(1  2 x )(1  2 x )
12. 9 x 2  16 y 2  (3 x  4 y )( 3 x  4 y )
 ( x  2)( x  3)
2
14. x  5 x  6  ( x  2)( x  3)
13. x  5 x  6
2
15. x  7 x  12  ( x  4)( x  3)
2
2
x
 4 x  21  ( x  7)( x  3)
16.
2
17. x  4 x  12  ( x  2)( x  6)
18. x  8 x  12  ( x  6)( x  2)
2
2
19. 3 x  14 x  15  (3 x  5)( x  3)
20. 4 x 2  16 x  15
 (2 x  5)( 2 x  3)