Plot 2D & 3D - Variables Range

Hi everybody,

I have been very surprised that both plot commands don't accept
the limits of the variables range if they are written in fractional
form:

(%i14) plot2d(2*x,[x,-5,5])$

    is OK, while, if 5 -> 5/2

(%i15) plot2d(2*x,[x,-5,5/2])$

Bad Range
[x,-5,5/2] must be of the form [variable,min,max]
-- an error.  Quitting.  To debug this try DEBUGMODE(TRUE);

Of course the same happens using plot3d().

This behaviour is the same in Maxina 5.5l while Maple and Mathematica
have not this problem.

Greetings from Italy,

Paolo