Answer 1

To prove relation is an equivalence relation
We have to show three properties
1. Reflexive
(a,a) ∈ R

2. Symmetric
(a,b) ∈ R
⇒ (b,a) ∈ R 

3. Transitive
 (a,b) ∈ R and (b,c) ∈ R
⇒ (a,c) ∈ R
1. R is reflexive because 2 divides (a - a)∀a ∈ z∀a ∈ z

2. 2 divides a - b
therefore, 2 divides b - a hence, (b,a) ∈ R
R is symmetric

3. (a, b) ∈ R
(b, c) ∈ R
then a − b and b − c are divisible by 2.
Now, a − c = ( a − b ) + ( b − c) = a−c
so, a − c is divisible by 2.
Therefore, (a, c ) ∈ R
Therefore, R is an equivalence relation.