### 2.8.5 Numerator and denominator of a fraction : `f2nd fxnd`

`f2nd` (or `fxnd`) takes as argument a fraction and returns,
the list of the numerator and denominator of this fraction after simplification
(for rational fractions see 2.28.5).

Input :

`f2nd(42/12)`

Output :

`[7,2]`