### 2.21.21 Simplify and express preferentially with cosine : `trigcos`

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

`trigcos` simplifies 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 cosine.

Input :

`trigcos(sin(x)``^`

`4+cos(x)``^`

`2+1)`

Output :

`cos(x)``^`

`4-cos(x)``^`

`2+2`