Consider the following statements
(A) To obtain prime numbers less than 121, we have to reject all the multiples of 2, 3, 5 and 7.
(B) Every composite number less than 121 is divisible by a prime number less than 11.
(C) 173 is not a prime number.
(D) 7710312401 is divisible by 11.
Which of the statement(s) given above is/are correct?
Choose the correct answer from the options given below:
Consider the following statements
(A) To obtain prime numbers less than 121, we have to reject all the multiples of 2, 3, 5 and 7.
(B) Every composite number less than 121 is divisible by a prime number less than 11.
(C) 173 is not a prime number.
(D) 7710312401 is divisible by 11.
Which of the statement(s) given above is/are correct?
Choose the correct answer from the options given below:
Solution
To find prime numbers up to any number using the Sieve of Eratosthenes, reject multiples of primes up to the square root of that number.
Primes less than 11 are: 2, 3, 5, 7
The statement says to reject multiples of 2, 3, 5, and 7.
Statement (A) is correct.
Any composite number must have at least one prime factor that is less than or equal to its square root. If a composite number , at least one factor must be .
For numbers less than 121:
Every composite number less than 121 must have a prime factor less than 11.
Statement (B) is correct.
To check if 173 is prime, test divisibility by all primes up to
Primes to check: 2, 3, 5, 7, 11, 13
is odd, not divisible by 2
Sum of digits , not divisible by 3
Doesn't end in 0 or 5, not divisible by 5
, not divisible by 7
, not divisible by 11
, not divisible by 13
Since 173 is not divisible by any prime up to , the number 173 is prime.
Statement (C) is incorrect.
Divisibility rule for 11: Sum the digits at odd positions (1st, 3rd, 5th...) and sum the digits at even positions (2nd, 4th, 6th...). If the difference is 0 or divisible by 11, then the number is divisible by 11.
Number: 7710312401
Odd positions (1,3,5,7,9):
Even positions (2,4,6,8,10):
Difference:
Since the difference is 0, the number is divisible by 11.
Statement (D) is correct.
Statements (A), (B), and (D) are correct.
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