Problem solved :-D I have though that between (diff(u,x)).(diff(u,y)) dot is not needed but it is :-) 

I was not been able to do that. For Example, In one sheet (nw) I have some variables a and b both are long symbolic expressions derived here. In the second worksheet (nw) I have sol:=({a+b=10,a*b+5=1},{a,b}), a:=eval(a,sol), b:=eval(a,sol). How can I include the second worksheet and use the solution from second worksheet in the first one?  

 The second file is dependent on the first, because the sheet has variable in the first  one and after calculation I need to import the solution from the second one.

 I have exported second file to mpl and tried read "file.mpl" but it was not work. Could you give me some example pleasehow to do that?