Rational Numbers | FIO

Question 13

You are told that 13311331 is a perfect cube. Can you guess without factorisation what its cube root is? Similarly, guess the cube roots of 49134913, 1216712167, and 3276832768.

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Solution
Understand the Question

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 0,1,4,5,6,90, 1, 4, 5, 6, 9 give the same digit in the cube root.
    • Digits ending in 282 \leftrightarrow 8 and 373 \leftrightarrow 7 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 13311331 without factorisation.

Step 1 · Find the Unit's and Ten's Digits

Form groups of three digits from the right: Group 2: 1,Group 1: 331\text{Group 2: } 1, \quad \text{Group 1: } 331

Unit's digit: Group 1 (331) ends in 1    Unit’s digit=1\text{Group 1 } (331) \text{ ends in } 1 \implies \text{Unit's digit} = 1

Ten's digit: Group 2 is 1\text{Group 2 is } 1 13=11    Ten’s digit=11^3 = 1 \le 1 \implies \text{Ten's digit} = 1

13313=11\sqrt[3]{1331} = 11

Answer

(i) 1111

(ii) Guess the cube root of 49134913 without factorisation.

Step 1 · Find the Unit's and Ten's Digits

Form groups of three digits from the right: Group 2: 4,Group 1: 913\text{Group 2: } 4, \quad \text{Group 1: } 913

Unit's digit: Group 1 (913) ends in 3\text{Group 1 } (913) \text{ ends in } 3 Since 73=343 ends in 3    Unit’s digit=7\text{Since } 7^3 = 343 \text{ ends in } 3 \implies \text{Unit's digit} = 7

Ten's digit: Group 2 is 4\text{Group 2 is } 4 13=1and23=81^3 = 1 \quad \text{and} \quad 2^3 = 8 Since 134<23    Ten’s digit=1\text{Since } 1^3 \le 4 < 2^3 \implies \text{Ten's digit} = 1

49133=17\sqrt[3]{4913} = 17

Answer

(ii) 1717

(iii) Guess the cube root of 1216712167 without factorisation.

Step 1 · Find the Unit's and Ten's Digits

Form groups of three digits from the right: Group 2: 12,Group 1: 167\text{Group 2: } 12, \quad \text{Group 1: } 167

Unit's digit: Group 1 (167) ends in 7\text{Group 1 } (167) \text{ ends in } 7 Since 33=27 ends in 7    Unit’s digit=3\text{Since } 3^3 = 27 \text{ ends in } 7 \implies \text{Unit's digit} = 3

Ten's digit: Group 2 is 12\text{Group 2 is } 12 23=8and33=272^3 = 8 \quad \text{and} \quad 3^3 = 27 Since 2312<33    Ten’s digit=2\text{Since } 2^3 \le 12 < 3^3 \implies \text{Ten's digit} = 2

121673=23\sqrt[3]{12167} = 23

Answer

(iii) 2323

(iv) Guess the cube root of 3276832768 without factorisation.

Step 1 · Find the Unit's and Ten's Digits

Form groups of three digits from the right: Group 2: 32,Group 1: 768\text{Group 2: } 32, \quad \text{Group 1: } 768

Unit's digit: Group 1 (768) ends in 8\text{Group 1 } (768) \text{ ends in } 8 Since 23=8    Unit’s digit=2\text{Since } 2^3 = 8 \implies \text{Unit's digit} = 2

Ten's digit: Group 2 is 32\text{Group 2 is } 32 33=27and43=643^3 = 27 \quad \text{and} \quad 4^3 = 64 Since 3332<43    Ten’s digit=3\text{Since } 3^3 \le 32 < 4^3 \implies \text{Ten's digit} = 3

327683=32\sqrt[3]{32768} = 32

Answer

(iv) 3232

Common Mistakes
  • 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 22 have cube roots ending in 88, and numbers ending in 33 have cube roots ending in 77 (and vice versa).
  • Applicability: This estimation shortcut works only for numbers that are already known to be perfect cubes.

More questions in FIO

Q1

Which of the following numbers are not perfect squares?

(i) 2032 (ii) 2048 (iii) 1027 (iv) 1089

Q2

Which one among 64264^2, 1082108^2, 2922292^2, 36236^2 has last digit 4?

Q3

Given 1252=15625125^2 = 15625, what is the value of 1262126^2?

(i) 15625+12615625 + 126

(ii) 15625+26215625 + 26^2

(iii) 15625+25315625 + 253

(iv) 15625+25115625 + 251

(v) 15625+51215625 + 51^2

Q4

Find the length of the side of a square whose area is 441 m2441 \text{ m}^2.

Q5

Find the smallest square number that is divisible by each of the following numbers: 4, 9, and 10.

Q6

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.

Q7

How many numbers lie between the squares of the following numbers?

(i) 16 and 17 (ii) 99 and 100

Q8

In the following pattern, fill in the missing numbers:

12+22+22=3222+32+62=7232+42+122=132\begin{aligned} 1^2 + 2^2 + 2^2 &= 3^2 \\ 2^2 + 3^2 + 6^2 &= 7^2 \\ 3^2 + 4^2 + 12^2 &= 13^2 \end{aligned}

(a) 42+52+202=()24^2 + 5^2 + 20^2 = (\underline{\quad})^2

(b) 92+102+()2=()29^2 + 10^2 + (\underline{\quad})^2 = (\underline{\quad})^2

Q9

How many tiny squares are there in the following picture? Write the prime factorisation of the number of tiny squares.

Q10

Find the cube roots of 2700027000 and 1064810648.

Q11

What number will you multiply by 1323 to make it a cube number?

Q12

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.

Q13

You are told that 13311331 is a perfect cube. Can you guess without factorisation what its cube root is? Similarly, guess the cube roots of 49134913, 1216712167, and 3276832768.

Q14

Which of the following is the greatest? Explain your reasoning.

(i) 67366367^3 - 66^3

(ii) 43342343^3 - 42^3

(iii) 67266267^2 - 66^2

(iv) 43242243^2 - 42^2

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