### 2.21.20 Simplify and express preferentially with sine : `trigsin`

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

`trigsin` simplify this expression with the formula :

sin(*x*)2+cos(*x*)2=1, tan(*x*)=sin(*x*)/cos(*x*) and
tries to rewrite the expression only with sine.

Input :

`trigsin(sin(x)``^`

`4+cos(x)``^`

`2+1)`

Output :

`sin(x)``^`

`4-sin(x)``^`

`2+2`