what is the meaning of closure property of addition
Closure (mathematics) A set has closure under an operation if performance of that operation on members of the set always produces a member of the same set; in this case we also say that the set is closed under the operation. For example, the positive integers are closed under addition, but not under subtraction:   1 − 2  {\displaystyle 1-2}  is not a positive integer even though both 1 and 2 are positive integers. Another example is the set containing only zero, which is closed under addition, subtraction and multiplication (because   0 + 0 = 0  {\displaystyle 0+0=0} ,   0 − 0 = 0  {\displaystyle 0-0=0} , and   0 × 0 = 0  {\displaystyle 0\times {0}=0} ).