NCERT Book Class 8 Maths Chapter 8 Algebraic Expressions and Identities

ALGEBRAIC EXPRESSIONS AND IDENTITIES 93

CHAPTER
Algebraic Expressions
and Identities
8
8.1 Addition and Subtraction of Algebraic
Expressions
In earlier classes, we have already become familiar with what algebraic expressions
(or simply expressions) are. Examples of expressions are:
x + 3, 2y – 5, 3x2, 4xy + 7 etc.
In the earlier classes, we have also learnt how to add and subtract algebraic expressions.
For example, to add 7x2 – 4x + 5 and 9x – 10, we do
7x2 – 4x + 5
+ 9x – 10
7x2 + 5x – 5
Observe how we do the addition. We write each expression to be added in a separate
row. While doing so we write like terms one below the other, and add them, as shown.
Thus 5 + (–10) = 5 –10 = –5. Similarly, – 4x + 9x = (– 4 + 9)x = 5x. Let us take some
more examples.
Example 1: Add: 7xy + 5yz – 3zx, 4yz + 9zx – 4y , –3xz + 5x – 2xy.
Solution: Writing the three expressions in separate rows, with like terms one below
the other, we have
7xy + 5yz –3zx
+ 4yz + 9zx – 4y
+ –2xy – 3zx + 5x (Note xz is same as zx)
5xy + 9yz + 3zx + 5x – 4y
Thus, the sum of the expressions is 5xy + 9yz + 3zx + 5x – 4y. Note how the terms, – 4y
in the second expression and 5x in the third expression, are carried over as they are,
since they have no like terms in the other expressions.