### 2.21.13 Transform tan(x) into sin(x)/cos(x) : `tan2sincos`

`tan2sincos` takes as argument an expression containing
trigonometric functions.

`tan2sincos` replaces tan(*x*) by
sin(*x*)/cos(*x*), in this expression.

Input :

`tan2sincos(tan(2*x))`

Output :

`sin(2*x)/cos(2*x)`