answersLogoWhite

0

One way is to test all the prime numbers equal or less than Sqrt[251]. These are: {2,3,5,7,11,13}.

We can eliminate 2, 3, and 5 because the number isn't odd, it's digits don't sum to a multiple of 3, and the last digit isn't a 5 or a 0.

For 7, note that 252 = 7*36, so 7 cannot divide 251 because that would imply 7 divides 1.

For 11, note that 253 = 11*23, so 11 cannot divide 251 because that would imply 11 divides 2.

For 13, note that 13*19 = 247, so 13 cannot divide 251 because that would imply 13 divides 4.

Hence, 251 is prime

User Avatar

Wiki User

15y ago

Still curious? Ask our experts.

Chat with our AI personalities

FranFran
I've made my fair share of mistakes, and if I can help you avoid a few, I'd sure like to try.
Chat with Fran
DevinDevin
I've poured enough drinks to know that people don't always want advice—they just want to talk.
Chat with Devin
SteveSteve
Knowledge is a journey, you know? We'll get there.
Chat with Steve

Add your answer:

Earn +20 pts
Q: How 251 is a prime number?
Write your answer...
Submit
Still have questions?
magnify glass
imp