Solution:
A number is a perfect cube only when each factor in the prime factorization is grouped in triples. Using this concept, the smallest number can be identified.
(i) 81
81 = 3 × 3 × 3 × 3
= 33 × 3
Here, the prime factor 3 is not grouped as a triplet. Hence, we divide 81 by 3, so that the obtained number becomes a perfect cube.
Thus, 81 ÷ 3 = 27 = 33 is a perfect cube.
Hence the smallest number by which 81 should be divided to make a perfect cube is 3.
(ii) 128
128 = 2 × 2 × 2 × 2 × 2 × 2 × 2
= 23 × 23 × 2
Here, the prime factor 2 is not grouped as a triplet. Hence, we divide 128 by 2, so that the obtained number becomes a perfect cube.
Thus, 128 ÷ 2 = 64 = 43 is a perfect cube.
Hence the smallest number by which 128 should be divided to make a perfect cube is 2.
(iii) 135
135 = 3 × 3 × 3 × 5
= 33 × 5
Here, the prime factor 5 is not a triplet. Hence, we divide 135 by 5, so that the obtained number becomes a perfect cube.
135 ÷ 5 = 27 = 33 is a perfect cube.
Hence the smallest number by which 135 should be divided to make a perfect cube is 5.
(iv) 192
192 = 2 × 2 × 2 × 2 × 2 × 2 × 3
= 23 × 23 × 3
Here, the prime factor 3 is not grouped as a triplet. Hence, we divide 192 by 3, so that the obtained number becomes a perfect cube.
192 ÷ 3 = 64 = 43 is a perfect cube
Hence the smallest number by which 192 should be divided to make a perfect cube is 3.
(v) 704
704 = 2 × 2 × 2 × 2 × 2 × 2 × 11
= 23 × 23 × 11
Here, the prime factor 11 is not grouped as a triplet. Hence, we divide 704 by 11, so that the obtained number becomes a perfect cube.
Thus, 704 ÷ 11 = 64 = 43 is a perfect cube
Hence the smallest number by which 704 should be divided to make a perfect cube is 11.
☛ Check: NCERT Solutions for Class 8 Maths Chapter 7
Video Solution:
Find the smallest number by which each of the following numbers must be divided to obtain a perfect cube (i) 81 (ii) 128 (iii) 135 (iv) 192 (v) 704
NCERT Solutions for Class 8 Maths Chapter 7 Exercise 7.1 Question 3
Summary:
The smallest number by which each of the following numbers must be divided to obtain a perfect cube (i) 81 (ii) 128 (iii) 135 (iv) 192 (v) 704 are (i) 3, (ii) 2, (iii) 5, (iv) 3, and (v) 11
☛ Related Questions:
Find the smallest number by which 3087,33275,120393 must be divided so that quotient is a perfect cube
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Text Solution
Solution : Given:<br> The prime factor of `3087` are<br> `=>3087=3xx3xx7xx7xx7`<br> Since, `3` does not occur in triplet.<br> So, `3087` is not a perfect cube.<br> In order to make a perfect cube .We have to divided by `9`<br>