Page 63 - ICSE Math 7

Division of rational numbers Maths Info

To divide one rational number by another non-zero rational
number, we find the product of the first rational number with The product of a non-zero rational number
and its reciprocal is always 1. For example,
the reciprocal of the second. 3 –3
– × reciprocal of
For example, 8 8
–3 8 –3 –8 24
= × = × = = 1
3 ÷ –2 = 3 × reciprocal of –2 8 –3 8 3 24
–4 5 –4 5
= –3 × 5 = –3 × 5 = –15 = 15
4 –2 4 × –2 –8 8
Example 14: Find the value of:
3 1 2 –4 2 –7
(a) ÷ (b) –6 ÷ (c) ÷ (–2) (d) ÷
4 2 7 5 13 65
3 1 3 2 3 × 2 6 3
Solution: (a) ÷ = × = = = Try This
4 2 4 1 4 × 1 4 2
2 –6 7 –42 Find the value of:
(b) –6 ÷ = × = = –21 4 2 3
7 1 2 2 (a) ÷ (b) 4 ÷
–4 –4 (–2) –4 1 –4 4 2 8 5 8
(c) ÷ (–2) = ÷ = × = = = 2 3 –3
5 5 1 5 –2 –10 10 5 (c) 0 ÷ 1 (d) 2 ÷ 4
4
3
3
2 –7 2 65 2 × 65 130 –130 –10 (e) –1 ÷ (f) –2 ÷ 1
(d) ÷ = × = = = = 8 4 13 5
13 65 13 –7 13 × –7 –91 91 7
1
Example 15: The cost of 7 metres of cable wire is ` 225. Find the cost of wire per metre?
2
1
Solution: Cost of 7 metres of wire = ` 225
2

i.e., cost of 15 metres of wire = ` 225
2
Cost of 1 metre of wire = ` 225 ÷ 15 = ` 225 × 2 = ` 30
2 15

EXERCISE 4.2

1. Add. (a) –3 and –6 (b) 5 and 8 (c) –8 and 3
17 51 21 35 17 51
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2. Subtract. (a) –9 from 6 (b) 3 from –9 (c) 13 from 11
22 11 4 –4 48 36

3. Write the additive inverse of: (a) 3 (b) 6 (c) –11 (d) –7 1
13 –23 30 3

5
4. Write the reciprocal of: (a) (b) –27 (c) –17 (d) 2 1
4 13 39 7
1
2
1
5. Multiply. (a) × 3 (b) × –24 (c) 3 × –7 (d) 2 × 5
3 –7 8 13 11 19 3 14
1
6. Divide. (a) –6 ÷ –2 (b) 2 ÷ 5 1 (c) 7 ÷ –4 (d) –5 ÷ 7
13 5 25 13 65 8 4

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