How to find a counterexample for this closed statement???

Question

math...ugh, i know. whatever i should do my own homework, but seriously i've been pulling my hair out for this problem (not literally) i just don't understand counterexamples and all that stuff. it's way confusing.

here's the problem:

"The rational numbers are closed under the operation of taking the square root."

PLEASE HELP!

Answer 1

okay choose a random number

not 36!

maybe something like you know.. ten?

then square root [notice i said square root not square]

the number, ten, which should come out to about 3.1,

which would not be in the set,

therefore, the set is not closed or is closed.. i don't know mrs. jarvis is weird when she explains, like geeeeeeee.

anyways, there's your counterexample.