Question

write an counter example for the statement: all square roots are rational numbers

Answer 1

That is not true. For example, if you take PI . Rounded is 3.14. But if you were to do the square root of pi then it would be an IRRATIONAL number. so it's false. 

Answer 2

It's false because if you take the square root of a decimal number or a complex number, you're sure to find an irrational number.