Chapter 5 · ICSE Class 9 Mathematics (Selina Concise)

Factorisation

Factorise algebraic expressions using common factors, grouping, and standard identities like the difference of squares.

🧩 Chapter 5

Factorising the expression x squared minus 9 The expression x squared minus 9 shown factorised into (x minus 3) times (x plus 3), using the difference of squares identity x² − 9 ↓ (x − 3)(x + 3)
5.1

Common Factor and Grouping

Factorising by taking out common factors, or by grouping terms in pairs that share a common factor.

1

Factorise: 6x²y + 9xy²

3xy(2x+3y)

2

Factorise by grouping: ax+ay+bx+by

a(x+y)+b(x+y) = (a+b)(x+y)

5.2

Factorising Using Identities

The difference of squares identity a²−b²=(a+b)(a−b) is one of the most useful factorisation tools.

1

Factorise: x²−49

(x+7)(x−7)

2

Factorise: 9x²−16y²

(3x+4y)(3x−4y)

Practice

Exercises

1. Factorise: 4a²b+6ab²

2. Factorise by grouping: px+py+qx+qy

3. Factorise: x²−81

4. Factorise: 25x²−4y²

5. Factorise: x²+8x+16