@Preben Alsholm 
Thank you!!!

@Preben Alsholm Thank you! Now I can compare my final plots with the plots from your code!

I have a question: 

Do the plots (got from ForwardEuler and BackwardEuler) have to be similar? I thought that these plots had to be different (a little bit), and Crank Nikolson would be in the middle. If it is right? I mixed up everythng...

Thank you for your answers! They help me!

@Preben Alsholm 
test.zip

Check again, please!

@Preben Alsholm 
implicit_method_Dirichle_B.C..mws
explicit_method_Neumann_B.C..mws

I hope, it works.

Hello @Preben Alsholm !!!

I have taken the fisrt picture from WORD MS. It is not from the code in maple. I just wanted to give an explanation of my problem.
I edited the post above. 

My codes:
explicit_method_Neumann_B.C..mw
implicit_method_Dirichle_B.C..mw

I will be very grateful if You help me!

@Preben Alsholm Thank you very much!!! It works!