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For example, subtract 7a b – 2ab from 11a b + 3ab
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(11a b + 3ab) – (7a b – 2ab) = 11a b + 3ab – 7a b + 2ab [Changing sign ‘+’ to ‘–’ and ‘–’ to ‘+’]
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= (11 – 7)a b + (3 + 2)ab = 4a b + 5ab
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Example 4: (i) Add 5ab + 4xy – 3ax + 2 and –13xy – 3ab + 7bx + 9.
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(ii) Subtract a – b – a b from 3a – 4a b + 5b .
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Solution: (i) 5ab + 4xy – 3ax + 2 (ii) 3a – 4a b + 5b 4
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(+) – 3ab – 13xy + 7bx + 9 a – a b – b 4
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2ab – 9xy – 3ax + 7bx + 11 (–) (+) (+)
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2a – 3a b + 6b 4
Example 5: Subtract –2p + 3q – 3r from the sum of 4p – 2q + 6r and 3p + 8q – 2r.
Solution: By adding 4p – 2q + 6r and 3p + 8q – 2r, we get
4p – 2q + 6r + (3p + 8q – 2r) = 7p + 6q + 4r
Next, 7p + 6q + 4r – (–2p + 3q – 3r) = 7p + 6q + 4r + 2p – 3q + 3r
= (7p + 2p) + (6q – 3q) + (4r + 3r)

= 9p + 3q + 7r

Think and Answer

Ziva made a drawing which is in the shape of a triangle. The perimeter of the drawing
is 15a – 14b + 13c.
If the two sides of the drawing are 5a + 7b – 9c and 4a – 6b + 12c respectively, then find the third
side of the drawing.

Practice Time 8A

1. Write all the terms and coefficients of variable terms of the following algebraic expressions.
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(a) 9ab – 5b + 1 (b) 11xyz – 6yz + xy (c) 7m n + 8mn – 4mn – 3
2. Identify the variable terms and write their factors in the following algebraic expressions.
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(a) x – x + 10 (b) 8yz – 2y + 7 (c) 8p + q – 2r + 5
3. Select the like and unlike terms in the given set of algebraic expressions.
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24x y, –7yx , 55x y, 17y x , 99y x
4. Classify the following algebraic expressions as monomials, binomials, and trinomials.
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(a) 2x + 7x (b) –145 (c) xy + z (d) 7x + y – 9
(e) 1 – 9mn – 9nm
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5. Write the degrees of the following algebraic expressions.
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3 3
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(a) 97d 54 (b) 7x – 4x + 8 (c) 1 – 6a + 5a – 8a 2 (d) 4x y + 5x y + 8
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3 3 3
(e) 2x y z + 5x y z + 8x yz + 7x y
Mathematics-8 198