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8th Grade > Mathematics

CUBES AND CUBE ROOTS MCQs

Total Questions : 57 | Page 4 of 6 pages
Question 31.


How many cubes of side length 3 can be fit in a cuboid of dimension 3×6×9?


  1.     2
  2.     6
  3.     9
  4.     10
 Discuss Question
Answer: Option B. -> 6
:
B

Volume of cuboid = 3×6×9 =162 cubic units


Volume of cube = 3×3×3= 27 cubic units


Number of cubes that can be fit in the cuboid is 16227 =  6


Question 32.


Find the number whose square is the sum of the cube of the first 5 natural numbers.


  1.     10
  2.     12
  3.     13
  4.     15
 Discuss Question
Answer: Option D. -> 15
:
D

The sum of first 5 natural numbers can be expressed as (n(n+1)2)2 where n = 5, which gives us
((5×6)2)2 =(302)2 = 152
Thus the cubes of first 5 natural numbers can be expressed as the square of 15.


Question 33.


How many perfect cubes are there between 0 and 100?


  1.     2
  2.     3
  3.     4
  4.     5
 Discuss Question
Answer: Option C. -> 4
:
C

Cube of 1 = 1


Cube of 2 = 8


Cube of 3 = 27


Cube of 4 = 64


So total 4 numbers have their cubes that lie between 0 and 100


Question 34.


When the cube root of 8 is multiplied with the cube root of -8, the result which cannot be obtained is 


  1.     8
  2.     -8
  3.     4
  4.     -4
 Discuss Question
Answer: Option C. -> 4
:
A, B, and C

Cube root of 8 is 2 and cube root of -8 is -2. When they are multiplied together, the result is -4.


Question 35.


The cube root of 30 lies between 5 and 6.


  1.     True
  2.     False
  3.     4
  4.     5
 Discuss Question
Answer: Option B. -> False
:
B

The cube of 3 = 27


The cube of 4 = 64


Since 30 lies between 27 & 64 so its cube root must lie between 3 & 4.
Hence, the given statement is false.


Question 36.


The cube of 0 is 


___
 Discuss Question
Answer: Option B. -> False
:

Cube of 0 is 0 (anything multiplied to 0 is 0)


Question 37.


What is the value of  38138 ?


  1.     2
  2.     2.5
  3.     0.5
  4.     1
 Discuss Question
Answer: Option B. -> 2.5
:
B

8= 2 x 2 x 2


Cube root of 8 = 2


So 2+  12 = 2.5


Question 38.


Which of the following is a perfect cube?


  1.     532
  2.     729
  3.     827
  4.     943
 Discuss Question
Answer: Option B. -> 729
:
B

By prime factorization, we get 729 = 3×3×3×3×3×3=36
So cube root of 729=(729)13=363=32=9
Since, the cube root of 729 is a natural number, 729 is a perfect cube.


Question 39.


a3 - b3 is always divisible by:


  1.     a - b
  2.     a × b
  3.     a + b
  4.     2a + b 
 Discuss Question
Answer: Option A. -> a - b
:
A

Take a=2 and b=1 and check.
Alternatively, we know the algebraic identity,


a3b3=(ab)(a2+2ab+b2)


 Hence , we can see that (ab) is a factor of a3 - b3
Thus, a3 - b3 is divisible by (ab)


Question 40.


What is the minimum number that should be multiplied with 360 in order to make it a perfect cube ?


  1.     75
  2.     70
  3.     65
  4.     80
 Discuss Question
Answer: Option A. -> 75
:
A

For a number to be a perfect cube, its prime factors should be in the form of triplets i.e. every prime factor should be raised to the power in multiples of 3
Prime factorizing the given number, we get 360=2×2×2×3×3×5
Hence, we can see that the given number must be multiplied by 3×5×5 i.e.75


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