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A number is divisible by 12 if it is divisible by:
2 and 6
3 and 4
2 and 3
1 and 12
3 and 4
For a number to be divisible by a composite number, it must be divisible by the prime factors of its co-prime pair. Since 3 and 4 have no common factor other than 1, divisibility by both ensures divisibility by their product.
A number needs to be divisible by 12.
12=3├Ч4,┬аwhere┬аgcd(3,4)=1
For a number to be divisible by a composite number, it must be divisible by the prime factors of its co-prime pair. Since 3 and 4 have no common factor other than 1, divisibility by both ensures divisibility by their product.
Many students select 2 and 6 because 2├Ч6=12; however, 2 and 6 are not co-prime (gcd(2,6)=2), meaning a number like 6 is divisible by 2 and 6 but not by 12.
Analyze the Composite Number
To check divisibility by a composite number n, we decompose it into factors a and b such that n=a├Чb and gcd(a,b)=1.
12=22├Ч3
Evaluate Option A
Factors 2 and 6 are not co-prime as they share a common factor of 2. For example, 6 is divisible by both 2 and 6, but 6 is not divisible by 12.
gcd(2,6)=2юАа=1
Evaluate Option B
Factors 3 and 4 are co-prime as their greatest common divisor is 1. Therefore, any number divisible by both 3 and 4 must be divisible by 3├Ч4=12.
gcd(3,4)=1
Final Verification
Since 3 and 4 satisfy the condition of being co-prime factors whose product is 12, the condition for divisibility is met.
12=3├Ч4
B is correct because 3 and 4 are co-prime factors of 12, satisfying the divisibility rule for composite numbers.
This concept is essential for finding HCF and LCM; always ensure factors used for divisibility checks are co-prime (e.g., for 18, check 2 and 9, not 3 and 6).