Proof that $(a, b) \mathrel{R} (c, d)$ iff $ad = bc$ is an equivalence relation Let $X = \{(a,b) \mid a,b \in \Bbb Z; b \ne 0\}$. We define $(a,b)\mathrel R (c,d)$ iff $ad = bc$. Prove that $R$ is an equivalence relation on the set $X$. Which known set do the equivalence classes of the relation form?

Any help on solving this please?

You need to show that the given relation is 


*

*reflexive For all $(a, b) \in X$, $ab = ba$. This is clearly true. Hence R is reflexive.

*symmetric  For all $(a, b), (c, d) \in X$, suppose $(a, b) R(c, d).$ Then $ad = bc $ if and only if $cb = da$ if and only if $(c, d) R (a,b)$. Therefore, R is 
symmetric.

*transitive Now, see what you can do with the following: Take arbitrary $(a, b), (c, d), (e, f)$ and assume $(a, b)R (c, d)$, and $(c, d) R (e, f)$. So 
$$ad = bc,\quad \text{and} \quad cf = de$$ Now, we use a little algebra to show that this implies $af = be$, and hence $(a, b) R (e, f)$.  $ad = bc \iff adf = bcf = b(cf).$ Also, we have $cf = de$. So $adf = b(de) \iff af = be$, as desired!
The relation has all three properties, and hence, is by definition, an equivalence relation.

Hint: $$(a, b) R (c, d) \;\text{ if and only if }\;\frac{a}{b} = \frac{c}{d}\; \text{ if and only if } \;ad = bc, \;\;(b\neq 0, d\neq 0)$$
