$y = 2 \operatorname { sen } ( x )$ 

$\quad y = \operatorname { cos } ( 2 x )$ 

$y = - 1 + 2 \operatorname { cos } ( x )$ 

$y = - 1 + 2 \operatorname { sen } ( x + \frac { \pi } { 2 } )$