**Definition of congruence:**

`a equiv b(mod m) <=> m|(a-b)`

**(i) Reflexivity**

`a equiv a(mod m)<=> m|(a-a) <=>m|0` which is true because 0 is divisible by any number. And that proves reflexivity

**(ii) Transitivity**

If `a equiv b(mod m)` and `b equiv c(mod m)`, then `m|(a-b)` and `m|(b-c)` which means that `m|((a-b)+(b-c))`. So `m|(a-c)` hence `a equiv c(mod m)` which proves transitivity

**(iii) Symmetry**

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**Definition of congruence:**

`a equiv b(mod m) <=> m|(a-b)`

**(i) Reflexivity**

`a equiv a(mod m)<=> m|(a-a) <=>m|0` which is true because 0 is divisible by any number. And that proves reflexivity

**(ii) Transitivity**

If `a equiv b(mod m)` and `b equiv c(mod m)`, then `m|(a-b)` and `m|(b-c)` which means that `m|((a-b)+(b-c))`. So `m|(a-c)` hence `a equiv c(mod m)` which proves transitivity

**(iii) Symmetry**

`a equiv b(mod m)<=>m|(a-b)<=>m|-(b-a)<=>m|(b-a)<=>b equiv a(mod m)`

And that proves symmetry.

Now by definition equivalence relation is reflexive, transitive and symmetric and since we have proven all three of these properties that means congruence modulo m is **an equivalence relation**.