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Which of the following statements are true?
(a) If a number is divisible by 3, it must be divisible by 9.
(b) If a number is divisible by 9, it must be divisible by 3.
(c) A number is divisible by 18, if it is divisible by both 3 and 6.
(d) If a number is divisible by 9 and 10 both, then it must be divisible by 90.
(e) If two numbers are co-primes, at least one of them must be prime.
(f) All numbers which are divisible by 4 must also be divisible by 8.
(g) All numbers which are divisible by 8 must also be divisible by 4.
(h) If a number exactly divides two numbers separately, it must exactly divide their sum.
(i) If a number exactly divides the sum of two numbers, it must exactly divide the two numbers separately.
(a) False
6 is divisible by 3, but not by 9.
(b) True, as 9 = 3 × 3
Therefore, if a number is divisible by 9, then it will also be divisible by
3.
(c) False
30 is divisible by 3 and 6 both, but it is not divisible by 18.
(d) True, as 9 × 10 = 90
Therefore, if a number is divisible by 9 and 10 both, then it will also be divisible by 90.
(e) False
15 and 32 are co-primes and also composite.
(f) False
12 is divisible by 4, but not by 8.
(g) True, as 8 = 2 × 4
Therefore, if a number is divisible by 8, then it will also be divisible by 2 and 4.
(h) True
2 divides 4 and 8 as well as 12. (4 + 8 = 12)
(i) False
2 divides 12, but does not divide 7 and 5.
Here are two different factor trees for 60. Write the missing numbers.
(a) As 6 = 2 × 3 and 10 = 5 × 2
(b) As 60 = 30 × 2, 30 = 10 × 3, and 10 = 5 × 2
Which factors are not included in the prime factorization of a composite number?
1 and the number itself
Write the greatest 4-digit number and express it in terms of its prime factors.
Greatest 4 digit number is 9999
Prime factors - 3x3x11x101
Write the smallest 5-digit number and express it in the form of its prime factors.
Smallest 5 digit number - 10000
Prime factors - 2x2x5x5x2x25x5
Find all prime factors of 1729 and arrange them in ascending order. Now state the relation, if any; between two consecutive prime factors.
1729 = 7 × 13 × 19
13 − 7 = 6, 19 − 13 = 6
The difference of two consecutive prime factors is 6.
The product of three consecutive numbers is always divisible by 6. Verify this statement with the help of some examples.
2 × 3 × 4 = 24, which is divisible by 6
9 × 10 × 11 = 990, which is divisible by 6
20 × 21 × 22 = 9240, which is divisible by 6
The sum of two consecutive odd numbers is divisible by 4. Verify this statement with the help of some examples.
Two consecutive odd number is 3 and 5
Sum = 3+5 = 8 (divisible by 4)
Other is 9+11 = 20 ( divisible by 4)
7+9 = 16 (divisible by 4)
In which of the following expressions, prime factorization has been done?
(a) 24 = 2 × 3 × 4 (b) 56 = 7 × 2 × 2 × 2
(c) 70 = 2 × 5 × 7 (d) 54 = 2 × 3 × 9
(a) 24 = 2 × 3 × 4
Since 4 is composite, prime factorisation has not been done.
(b) 56 = 7 × 2 × 2 × 2
Since all the factors are prime, prime factorisation has been done.
(c) 70 = 2 × 5 × 7
Since all the factors are prime, prime factorisation has been done.
(d) 54 = 2 × 3 × 9
Since 9 is composite, prime factorisation has not been done.
Determine if 25110 is divisible by 45.
[Hint: 5 and 9 are co-prime numbers. Test the divisibility of the number by 5 and 9].
45 = 5 × 9
Factors of 5 = 1, 5
Factors of 9 = 1, 3, 9
Therefore, 5 and 9 are co-prime numbers.
Since the last digit of 25110 is 0, it is divisible by 5.
Sum of the digits of 25110 = 2 + 5 + 1 + 1 + 0 = 9
As the sum of the digits of 25110 is divisible by 9, therefore, 25110 is divisible by 9.
Since the number is divisible by 5 and 9 both, it is divisible by 45.
18 is divisible by both 2 and 3. It is also divisible by 2 × 3 = 6. Similarly, a number is divisible by both 4 and 6. Can we say that the number must also be divisible by 4 × 6 = 24? If not, give an example to justify our answer:
No. It is not necessary because 12 and 36 are divisible by 4 and 6 both, but are not divisible by 24.
I am the smallest number, having four different prime factors. Can you find me?
Since it is the smallest number of such type, it will be the product of 4 smallest prime numbers.
2 × 3 × 5 × 7 = 210
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