Is 1339 prime

Answer 1

By inspection, it does not have 2, 3, 5, or 9 as a factor. So take the square root and you will need to check numbers up to and including the square root. If you don't have a calculator with square root handy, then notice that the number is less than 1600 (1600 = 16 x 100, and sqrt(16)=4, and sqrt(100) = 10) so the square root of 1339 is less than (4 x 10 = 40). You will only need to check odd numbers, and we already eliminated 3 and 5, so start with 7. When you get to 13, 1339/13 = 103, so you can stop, and 1339 is not prime.