Hello, Carlos!

If a rectangle is laid flat and a circle is just touching the rectangle, what shape would you draw

if you put a pencil in the centre of the circle and drew around the rest of the rectangle?

If I understand the problem, we are rolling a circle around a rectangle

. . and tracing the path of its center.

The figure would be a "rectangle" with rounded corners.

The shaded region is the given rectangle.

$\displaystyle R$ is the radius of the circle.

Code:

* * * * *
* : : *
* :R : *
* : : *
: :
* - - - o---------------o - - - *
* R |:::::::::::::::| *
* |:::::::::::::::| *
* |:::::::::::::::| *
* |:::::::::::::::| *
* |:::::::::::::::| *
* |:::::::::::::::| *
* - - - o---------------o - - - *
: : R
* : : *
* : R: *
* : : *
* * * * *