Composite number Problem | B.Stat Objective | TOMATO 75
Try this TOMATO problem from I.S.I. B.Stat Entrance Objective Problem based on Sequence and composite number.
Composite number Problem (B.Stat Objective)
Consider the sequence \(a_1\)=101, \(a_2\)=10101,\(a_3\)=1010101 and so on. Then \(a_k\) is a composite number ( that is not a prime number)
- if and only if \(k \geq 2\) and \(11|(10^{k+1}+1)\)
- if and only if \(k \geq 2\) and k-2 is divisible by 3
- if and only if \(k \geq 2\) and \(11|(10^{k+1}-1)\)
- if and only if \(k \geq 2\)
Key Concepts
Logic
Sequence
Composite number
Check the Answer
Answer: if and only if \(k \geq 2\) and k-2 is divisible by 3
B.Stat Objective Question 75
Challenges and Thrills of Pre-College Mathematics by University Press
Try with Hints
for \(a_k\) \(k \geq 2\) may be prime also then not considering this here
for \(a_{8}\) \(10^{9}-1\) and \(10^{9}+1\) not divisible by 11
8-2 is divisible by 3 and \(a_{8}\) is composite number then \(a_{k}\) is composite if and only if \(k \geq 2\) and k-2 is divisible by 3.
Other useful links
- https://cheenta.com/rational-number-and-integer-prmo-2019-question-9/
- https://www.youtube.com/watch?v=lBPFR9xequA
Related Program
I.S.I. & C.M.I. Entrance Program
Taught by students and alumni of I.S.I. & C.M.I.