You are right, I mean plot and you are right about your rephrasing. 

What I want is to mark the solutions in the plot; like Markiyan Hirnyk 3963 did it in the picture 

You are right, I mean plot and you are right about your rephrasing. 

What I want is to mark the solutions in the plot; like Markiyan Hirnyk 3963 did it in the picture 

@Markiyan Hirnyk 

I mean diameter

thank you a lot!!!!!!

thank you a lot!!!!!!

Hi, THANK you for your answer. Is it possible to fill this rectangle with color, like you fill a triangle with color:

> restart;
> with(geometry);
> point(a, 0, 0), point(b, 2, 0); point(c, 2, -2), point(d, 0, -2);
%;
 

> square(Sq, [a, b, c, d]); draw(Sq(color = "Grey"), filled = true);

Hi, THANK you for your answer. Is it possible to fill this rectangle with color, like you fill a triangle with color:

> restart;
> with(geometry);
> point(a, 0, 0), point(b, 2, 0); point(c, 2, -2), point(d, 0, -2);
%;
 

> square(Sq, [a, b, c, d]); draw(Sq(color = "Grey"), filled = true);