Question: Maple doesn't simplify equation?

Hi all,

 

I have a strange problem with the Classic worksheet in maple 15. To explain the problem i added an image.

when i want to plot vergl_6 without copying the equation i get an error. But when i just copy and paste equation 6 like i did everything works like i suspect.

Can anybody tell me the problem? I need to work with this equation much further and with different parameters, so i can't copy and paste it every time...

 

thanks in advance!

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