Question: problem with Programming

Please look at this codes!!!

i cant understand the idea of how maple works with parameters...

Problem.JPG

if u watch it exactly u can see that first maple understands that diff(a__1,t) is equall 1; but anfter that it couldnt load the value 1.
why?

what is hapening?

what can i do for this problem. its not the main code. i just send u an example of what  i want and what maple does. plz help me. tnx 

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