Prime numbers

Question

is there an easier way to find out how many prime numbers are under 100 without going through each number? Thanks!

Answer 1

2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97....prime numbers up to 100.

 

To answer your question....not really. Sorry if that disappoints you but here are a few tips.

 

If a number can't be divided by any prime number less than or equal
to the square root of the number, then it is prime.

It means that we can find all the primes under 100 by checking for
divisibility by the primes 2, 3, 5, and 7. The number 11 is no good because

the square root of 100 is 10 and 11 is higher than that.

 

Numbers ending in 2 are ruled out b/c they are divisible by 2.

 

Numbers ending in 0 or 5 are ruled out b/c they are divisible by 5.

(But remember, 5 is a prime number)

 

So far then, all other primes less than 100 must end in 1, 3, 7, or 9.

And lastly, a number is divisible by 3 if the sum of its digits is divisible by
3, so we can rule those numbers out.

 

Hope this helps.