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For $\alpha \in N$, consider a relation $R$ on $N$ given by $R =\{( x , y ): 3 x +\alpha y$ is a multiple of 7$\}$.The relation $R$ is an equivalence relation if and only if.
$\alpha=14$
$\alpha$ is a multiple of $4$
$4$ is the remainder when $\alpha$ is divided by $10$
$4$ is the remainder when $\alpha$ is divided by $7$
Solution
For $R$ to be reflexive $\Rightarrow xRx$
$\Rightarrow 3 x +\alpha x =7 x \Rightarrow(3+\alpha) x =7 K$
$\Rightarrow 3+\alpha=7 \lambda \Rightarrow \alpha=7 \lambda-3=7 N +4, K , \lambda, N \in I$
$\therefore$ when $\alpha$ divided by $7$, remainder is $4$.
$R$ to be symmetric $x R y \Rightarrow y R x$
$3 x +\alpha y =7 N _{1}, 3 y +\alpha x =7 N _{2}$
$\Rightarrow(3+\alpha)( x + y )=7\left( N _{1}+ N _{2}\right)=7 N _{3}$
Which holds when $3+\alpha$ is multiple of $7$
$\therefore \alpha=7 N +4 \text { (as did earlier) }$
$R$ to be transitive
$xRy \& yRz \Rightarrow xRz$.
$3 x +\alpha y =7 N _{1} \& 3 y +\alpha z =7 N _{2}$
$3 x +\alpha z =7 N _{3}$
$\therefore 3 x +7 N _{2}-3 y =7 N _{3}$
$\therefore 7 N _{1}-\alpha y +7 N _{2}-3 y =7 N _{3}$
$\therefore 7\left( N _{1}+ N _{2}\right)-(3+\alpha) y =7 N _{3}$
$\therefore(3+\alpha) y =7\,N$
Which is true again when $3+\alpha$ divisible by $7$ when $\alpha$ divided by $7$, remainder is $4$ .