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Published on: 14/09/2019
Cubes and Cube Roots
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Questions + Answers key
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1.
Is 1188 a perfect cube? If not, by which smallest natural number should 1188 be divided so that the quotient is a perfect cube?
2.
Is 392 a perfect cube? If not, find the smallest natural number by which it must be multiplied so that the product is a perfect cube.
3.
Which of the following are the cubes of odd numbers?
(a) 1331
(b) 4096
(c) 5832
(d) 8000
(e) 3375
(f) 4913
4.
Find the cube root of 1728.
5.
Find the smallest number that must be added to 210 so that itbecomes a perfect cube.
6.
The volume of a cubical box is 13.824 m3. then find the length of each side of the box.
7.
Is 9261 a perfect cube? If yes, find that number.
8.
Find the cube root of - 0.512 .
9.
Find the cube root of 19683 by factorisation method.
10.
Is 150 a perfect cube?
11.
Find the cube root of each of the following numbers by prime factorisation method.
175616
12.
Consider the following pattern.
23-13= 1 + 2 x 1 x 3, 33-23=1 + 3 x 2 x 3, 43-33=1 + 4 x 3 x 3
Using the above pattern, find the value of the following:
73 - 63
13.
Find the one's digit of the cube of each of the following numbers.
53
14.
Find the one's digit of the cube of each of the following numbers.
8888
15.
Is 2744 a perfect cube?
1.
1188 = 2 x 2 x 3 x 3 x 3 x 11
The primes 2 and 11 do not appear in groups of three. So, 1188 is not a perfect cube. In the factorisation of 1188 the prime 2 appears only two times and the prime 11 appears once. So, if we divide 1188 by 2 x 2 x 11 = 44, then the prime factorisation of the quotient will not contain 2 and 11.
Hence the smallest natural number by which 1188 should be divided to make it a perfect cube is 44.
2.
392 = 2 x 2 x 2 x 7 x 7
The prime factor 7 does not appear in a group of three. Therefore, 392 is not a perfect cube. To make its a cube, we need one more 7. In that case
392 x 7 = 2 x 2 x 2 x 7 x 7 x 7 = 2744
Hence the smallest natural number by which 392 should be multiplied to make a perfect cube is 7.
3.
(a) 1331
(e) 3375
(f) 4913
4.
By prime factorisation, we have


5.
6
6.
2.4 m
7.
21
8.
-0.8
9.
27
10.
Given, number is 150.
Now, prime factorisation of 150
150 = 2 x 3 x 5 x 5
The prime factors of 150 do not appear in group of triples.
Hence, 150 is not a perfect cube.

11.
By prime factorisation,
we have
175616 = 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 7x 7 x 7 = 2 x 2 x 2 x 7 = 56

Thus, the cube root of 175616 is 56.
12.
73 - 63 = 1+ 7 x 6 x 3 = 1+ 7 x 18 = 1+ 126 = 127
13.
We have, 53
One's digit of 53 = 3
Now, cube of one's digit of 53 = (3)3 = 3 x 3 x 3 = 27
Hence, the one's digit in the cube of 53 is 7.
14.
We have, 8888
One's digit of 8888 = 8
Now, cube of the one's digit of 8888 = (8)3 = 8 x 8 x 8 =512
Hence,one's digit in the cube of 8888 is 2.
15.
We have, 2744
Resolving 2744 into prime factors, We get
2744= 2 x 2 x 2 x 7 x 7 x 7 =(2 x 7)3=(14)3
Clearly, prime factor of2744 are grouped into triplets
So,2744 is a perfect cube of 14.

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