Equality of Rational Numbers

Theorem
Let $a, b, c, d$ be integers, with $b$ and $d$ nonzero.


 * $(1) : \quad$ The rational numbers $\dfrac ab$ and $\dfrac cd$ are equal.
 * $(2) : \quad$ The integers $ad$ and $bc$ are equal.

Proof
Note that by definition, $\Q$ is the quotient field of $\Z$.

1 implies 2
Let $\dfrac ab = \dfrac cd$ in $\Q$.

Then $bc = ad$ in $\Q$.

By Canonical Mapping to Quotient Field is Injective, $bc = ad$ in $\Z$.

2 implies 1
Let $bd = ac$ in $\Z$.

By definition of ring homomorphism, $bc = ad$ in $\Q$.

Thus Let $\dfrac ab = \dfrac cd$ in $\Q$.