All, thank you for your help. I have made progress.

I will continue working on it after a few days.

@Kitonum 

Thanks for your advice. I have used index. 
The whole formula i made (when i simplify it a bit)
F(y):=A+B+C;
A:=10*y;
B:=y²-4;
C:=5-y;
F(y):=F(y); Gives me --> F(y):=A+B+C; Instead of F(y):=10*y+y²-4+5-y

When i use index [] instead of () i get the same result.

@acer 

Thanks a lot for your answer. I have tried your option in a new worksheet. Just with a simple formula. But I think you misunderstood you.

if i have a realy simple formula:

f:=5+x;
                           f := 5 + x
f(5)
                            5 + x(5)
 

It doesnt give me what you tell me, so i think i misunderstood. Could you give me an example like that? Then I will change all formula's into something better ;) (I rather not use the unapply)

The error that bothers me the most is the difference between M_Inwendig1(y) and M_Inwendig2(y). I botf cases i do exactly the same, but at M_Inwendig2(y) it fails.
When i turn botf of them into M_Inwendig1[y] and M_Inwendig2[y] still M_Inwendig2[y] fails.

In the past i have given them the same "type" of name: Wis_BS2_Taak1_2019-2020.mw
Everything works just fine. I expected, if I do the same, I will get the same result...

@acer 

Thanks, it worked. I changed it so the white part is now marked.

PW := piecewise(
          x>=x[el] and x<x1[pl], Re(Oplossing111(x)),
          x>=x1[pl] and x<2, Oplossing124(x),
          x>=2 and x<x2[pl], Oplossing224(x),
          x>=x2[pl] and x<x2[el], Re(Oplossing211(x)),
          0.105):
display(
  plots:-shadebetween(PW,0.105, x=1.7..2.3, transparency=0.8),
plots:-shadebetween(-PW,-0.105, x=1.7..2.3, transparency=0.8),
 

Wis_BS2_Taak2_2019-2020_20200319-2.mw

Hello,

It was my intention not to upload it, i just need the code to fill-up. I am not quite sure why you would need the document/code.

But here you go.

It is my assignment for math: calculate the Elastoplastich fase of a steel I profile.

Regards,