Question
A binary operation * on the set {0, 1, 2, 3, 4, 5} is defined as:

 a * b = $ \begin{matrix} \text{a + b} & \text{if} & \text{a + b < 6} \\ \text{a + b - 6,} & \text{if} & \text{a + b }\geq6 \\ \end{matrix}$.

Show that zero is the identity for this operation and each element 'a' of the set is, invertible with 6 – a, being the inverse of 'a'.

Answer

since a * 0 = a + 0 = a 

and 0 * a = 0 + a = a

Note: $\forall$ $\in$ {0, 1, 2, 3, 4, 5}

$\therefore$ 0 is the identity for *.

Also,$\forall$ a $\in$ {0, 1, 2, 3, 4, 5}, a * (6 – a) = a + (6 – a) – 6

= 0 (which is identity)

Each element ‘a’ of the set is invertible with (6 – a), being the inverse of ‘a’.

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