Question
Determine whether the following operations define a binary operation on the given set or not:
'+6' on S = {0, 1, 2, 3, 4, 5} defined by, $\text{a}+_6\text{b}=\begin{cases}\text{a}+\text{b},&\text{if a}+\text{b}<6\\\text{a}+\text{b}-6,&\text{if a}+\text{b}\geq6\end{cases}$

Answer

We have,

S = {0, 1, 2, 3, 4, 5}

and, $\text{a}+_6\text{b}=\begin{cases}\text{a}+\text{b},&\text{if a}+\text{b}<6\\\text{a}+\text{b}-6,&\text{if a}+\text{b}\geq6\end{cases}$

Let $\text{a}\in\text{S}$ and $\text{b}\in\text{S}$ such that a + b < 6

Then $\text{a}+_6\text{b}=\text{a}+\text{b}\in\text{S}$ $\big[\because$ a + b < 6 = 0, 1, 2, 3, 4, 5$\big]$

Let $\text{a}\in\text{S}$ and $\text{b}\in\text{S}$ such that a + b > 6

Then $\text{a}+_6\text{b}=\text{a}+\text{b}-6\in\text{S}$ $\big[\because\ \text{if a}+\text{b}\geq6$ then $\text{a}+\text{b}-6\geq6$ = 0, 1, 2, 3, 4, 5$\big]$

$\therefore\ \text{a}+_6\text{b}\in\text{S}$ for $\text{a, b}\in\text{S}$

$\therefore$ +6 defined a binary operation on S.

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