There cannot be a counterexample since the assertion is true.
This requires you to accept the true fact that the terminating decimal 1.25, for example, is equivalent to the repeating decimal 1.25000... (or even 1.24999.... ).
There is no counterexample because the set of whole numbers is closed under addition (and subtraction).
no
No. A number cannot be closed under addition: only a set can be closed. The set of rational numbers is closed under addition.
Quite simply, they are closed under addition. No "when".
-3 is a negative integer. The absolute value of -3 is +3 which is not a negative integer. So the set is not closed.
Division by 0, which can also be written as 0.000... (repeating) is not defined.
In fact, the statement is true. Consequently, there is not a proper counterexample. The fallacy is in asserting that a terminating decimal is not a repeating decimal. First, there is the trivial argument that any terminating decimal can be written with a repeating string of trailing zeros. But, Cantor or Dedekind (I can't remember which) proved that any terminating decimal can also be expressed as a repeating decimal. For example, 2.35 can be written as 2.3499... Or 150,000 as 149,999.99... Thus, a terminating decimal becomes a recurring decimal. As a consequence, all real numbers can be expressed as infinite decimals. And that proves closure under addition.
There is no counterexample because the set of whole numbers is closed under addition (and subtraction).
no
No. A number cannot be closed under addition: only a set can be closed. The set of rational numbers is closed under addition.
The set of even numbers is closed under addition, the set of odd numbers is not.
Quite simply, they are closed under addition. No "when".
-3 is a negative integer. The absolute value of -3 is +3 which is not a negative integer. So the set is not closed.
2 = 2/1 is rational. Sqrt(2) is not rational.
-2 - (-5) = -2 + +5 = +3. (+3 is not in the set of negative numbers.)
That is correct, the set is not closed.
The numbers are not closed under addition because whole numbers, even integers, and natural numbers are closed.