Rational Numbers | FIO

Question 1

Which of the following numbers are not perfect squares?

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

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Solution
Understand the Question
  • A perfect square is a number formed by multiplying an integer by itself (e.g., 5×5=255 \times 5 = 25).
  • The unit digit of squares of digits 00 to 99 are:
    • 02=0, 12=1, 22=4, 32=9, 42=16, 52=25, 62=36, 72=49, 82=64, 92=810^2 = 0,\ 1^2 = 1,\ 2^2 = 4,\ 3^2 = 9,\ 4^2 = 16,\ 5^2 = 25,\ 6^2 = 36,\ 7^2 = 49,\ 8^2 = 64,\ 9^2 = 81
  • A perfect square can only end in 0,1,4,5,6,0, 1, 4, 5, 6, or 99.
  • Any number ending in 2,3,7,2, 3, 7, or 88 cannot be a perfect square.

(i) 2032

Step 1 · Check the Unit Digit of 2032

The unit digit of 20322032 is 22.

Since a perfect square cannot end in 22, 20322032 is not a perfect square.

Answer

(i) Not a perfect square

(ii) 2048

Step 1 · Check the Unit Digit of 2048

The unit digit of 20482048 is 88.

Since a perfect square cannot end in 88, 20482048 is not a perfect square.

Answer

(ii) Not a perfect square

(iii) 1027

Step 1 · Check the Unit Digit of 1027

The unit digit of 10271027 is 77.

Since a perfect square cannot end in 77, 10271027 is not a perfect square.

Answer

(iii) Not a perfect square

(iv) 1089

Step 1 · Verify if 1089 is a Perfect Square

The unit digit of 10891089 is 99, so it can be a perfect square.

Since 302=90030^2 = 900 and 402=160040^2 = 1600, the square root must be between 3030 and 4040.

A number ending in 99 can have a square root ending in 33 or 77, so the possibilities are 3333 or 3737.

Testing 3333:

33×33=(30+3)×(30+3)=30×30+30×3+3×30+3×3=900+90+90+9=1080+9=1089\begin{aligned} 33 \times 33 &= (30 + 3) \times (30 + 3) \\ &= 30 \times 30 + 30 \times 3 + 3 \times 30 + 3 \times 3 \\ &= 900 + 90 + 90 + 9 \\ &= 1080 + 9 \\ &= 1089 \end{aligned}

Since 332=108933^2 = 1089, 10891089 is a perfect square.

Answer

(iv) It is a perfect square (332=108933^2 = 1089)

Common Mistakes
  • Assuming all numbers ending in 0,1,4,5,6,90, 1, 4, 5, 6, 9 are perfect squares: A unit digit of 99 only indicates that a number might be a square (e.g. 1919 ends in 99 but is not a perfect square). Verification by calculation is necessary.
  • Not remembering impossible unit digits: A number ending in 2,3,7,2, 3, 7, or 88 is guaranteed not to be a perfect square and can be ruled out immediately without factorization.

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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