What property of rational numbers

(i) Closure Property :  The sum of any two rational numbers is always a rational number. This is called ‘Closure property of addition’ of rational numbers. Thus, Q is closed under addition

If a/b and c/d are any two rational numbers, then

(a/b) + (c/d) is also a rational number

Example :  2/9 + 4/9  =  6/9  =  2/3 is a rational number

(ii) Commutative Property :  Addition of two rational numbers is commutative.

If a/b and c/d are any two rational numbers, then

(a/b) + (c/d)  =  (c/d) + (a/b)

Example :  2/9 + 4/9  =  6/9  =  2/3 

(iii) Associative Property :Addition of rational numbers is associative.

If a/b, c/d and e/f  are any three rational numbers, then

a/b + (c/d + e/f)  =  (a/b + c/d) + e/f

Example :

2/9 + (4/9 + 1/9)  =  2/9 + 5/9  =  7/9 

(2/9 + 4/9) + 1/9  =  6/9 + 1/9  =  7/9 

So, 

2/9 + (4/9 + 1/9)  =  (2/9 + 4/9) + 1/9

(iv) Additive Identity :

The sum of any rational number and zero is the rational number itself.

If a/b is any rational number, then

a/b + 0  =  0 + a/b  =  a/b

Zero is the additive identity for rational numbers.

Example : 

2/7 + 0  =  0 + 2/7  =  2/7