While answering questions on Brainly, I ran into a lot of them that talked about a set being 'closed'. I didn't know what that means, so do ya know what I did ? I went and looked it up. I found it, and then I read about it, and now I know what it means !
A set that's 'closed under addition' means that if you add two numbers in the set, you get another number that's also in the set.
Let's see:
-- 3 is a prime number -- 5 is a prime number -- (3+5) = 8 -- 8 is NOT a prime number
-- We added two prime numbers and got a number that is NOT prime. So the set of prime numbers is NOT closed under addition.
