Question 13
You are told that is a perfect cube. Can you guess without factorisation what its cube root is? Similarly, guess the cube roots of , , and .
To find the cube root of a perfect cube by estimation (without prime factorisation):
- Step 1 (Make groups): Group the digits into sets of three starting from the unit's place (right to left). The rightmost group determines the unit's digit, and the remaining leftmost group determines the ten's digit.
- Step 2 (Unit's digit): Look at the last digit of the first group:
- Digits ending in give the same digit in the cube root.
- Digits ending in and interchange.
- Step 3 (Ten's digit): For the second group, find the largest single-digit integer whose cube is less than or equal to this number.
(i) Guess the cube root of without factorisation.
Step 1 · Find the Unit's and Ten's Digits
Form groups of three digits from the right:
Unit's digit:
Ten's digit:
(i)
(ii) Guess the cube root of without factorisation.
Step 1 · Find the Unit's and Ten's Digits
Form groups of three digits from the right:
Unit's digit:
Ten's digit:
(ii)
(iii) Guess the cube root of without factorisation.
Step 1 · Find the Unit's and Ten's Digits
Form groups of three digits from the right:
Unit's digit:
Ten's digit:
(iii)
(iv) Guess the cube root of without factorisation.
Step 1 · Find the Unit's and Ten's Digits
Form groups of three digits from the right:
Unit's digit:
Ten's digit:
(iv)
- Grouping Direction: Always start grouping 3 digits from the right (unit's end) to the left. Grouping from left to right gives incorrect group splits.
- Unit Digit Confusion: Remember the unit-digit pairs: numbers ending in have cube roots ending in , and numbers ending in have cube roots ending in (and vice versa).
- Applicability: This estimation shortcut works only for numbers that are already known to be perfect cubes.
More questions in FIO
Which of the following numbers are not perfect squares?
(i) 2032 (ii) 2048 (iii) 1027 (iv) 1089
Which one among , , , has last digit 4?
Given , what is the value of ?
(i)
(ii)
(iii)
(iv)
(v)
Find the length of the side of a square whose area is .
Find the smallest square number that is divisible by each of the following numbers: 4, 9, and 10.
Find the smallest number by which 9408 must be multiplied so that the product is a perfect square. Find the square root of the product.
How many numbers lie between the squares of the following numbers?
(i) 16 and 17 (ii) 99 and 100
In the following pattern, fill in the missing numbers:
(a)
(b)
How many tiny squares are there in the following picture? Write the prime factorisation of the number of tiny squares.
Find the cube roots of and .
What number will you multiply by 1323 to make it a cube number?
State true or false. Explain your reasoning.
(i) The cube of any odd number is even.
(ii) There is no perfect cube that ends with 8.
(iii) The cube of a 2-digit number may be a 3-digit number.
(iv) The cube of a 2-digit number may have seven or more digits.
(v) Cube numbers have an odd number of factors.
You are told that is a perfect cube. Can you guess without factorisation what its cube root is? Similarly, guess the cube roots of , , and .
Which of the following is the greatest? Explain your reasoning.
(i)
(ii)
(iii)
(iv)