@Carl Love 
Ah great , Thank you very much ! 

Now It's clear 

thank you for helping me and for your time @Carl Love @tomleslie  

@tomleslie 
Only one thing is a  little bit unclear is that line :
plot(expr, m=lhs(sol[1][1])..rhs(sol[1][2])) 
could please tell me for what reason you used it 

thanks 

@tomleslie 
Thank you very much again

I greatly appreciate the assistance you have provided me

Now I am able to make more precise 3d plots for u2 function scince i know the exact values of m 

Its a first week I start using Maple so with your tips and codes  I leard a lot 

Thank you ! 

with regards 
David 

@tomleslie  your code was very helpfull , thank you very much ! I think I used it correctly and finally got the right results . 

so to define the problem precisely :

I want to plot u2 , following function : ( which is periodic function ) 

but I have to define argument (eta)  of the function from the following inequality  (m=eta)

so what I did , i found values of m where inequality is correct ( actually i did it manually( using 2d plot) ? because when I was solving inequality answers were only negative) 

and then used that values of m=eta for the first equation to make 3d plot of u2 

I used the code you gave me and here is the final result  . I think it has to be ok 

Thank you very much again ! 

@Earl Ah great ! thanks a lot