Yes, they are.
The set of even numbers is closed under addition, the set of odd numbers is not.
Negative numbers are not closed under addition. When you add two negative numbers together, the result is always a negative number, which fits the definition of closure. However, if you add a negative number and a positive number, the result can be either positive, negative, or zero, thus violating the closure property for negative numbers. Therefore, negative numbers alone are not closed under addition.
The numbers are not closed under addition because whole numbers, even integers, and natural numbers are closed.
Quite simply, they are closed under addition. No "when".
No. A number cannot be closed under addition: only a set can be closed. The set of rational numbers is closed under addition.
Yes. The set of real numbers is closed under addition, subtraction, multiplication. The set of real numbers without zero is closed under division.
yes because real numbers are any number ever made and they can be closed under addition
Rational numbers are closed under addition, subtraction, multiplication. They are not closed under division, since you can't divide by zero. However, rational numbers excluding the zero are closed under division.
Addition.
Yes.
no it is not
no