Aptitude - Numbers - Discussion

We can find whether a given number is prime by using the formula n*n>=p

where n is the smallest number whose square is greater then or equal to p i:e the given number.

if p is divisible by any of the prime numbers less than n then it is not a prime otherwise it is a prime number

6k + 1 or 6k - 1 is a simplified form. Any prime number always satisfies this condition,where k can be 1,2,3...9.

But plz Note: Every prime no can be written in the form of 6k+1,6K-1 But every no which can be written in the form of 6k+1,6k-1 are not necessarily prime.

Here 91 can be written as 6k + 1 = 6(15) + 1 where k = 15,
But it is not prime as it is divisible by 13,7.

What is the meaning of " k " ? 6k+1 or 6k-1 please explain.

Here we have to notice that the above mentioned is not the general form of a prime number. There is no general form for prime no's, the above mentioned 6k+1 or 6k-1 will help you to find the prime no's but not for all the values of k.

I know that prime numbers are of the form either 6k-1 or 6K+1 . Here 91 is of the form 6k+1 but still it is prime how come?