Question: how to solve the differentiation in my work?

i have a problem of my equation with differentiation,in the first i run it total OK ,but when i input the value into the equation, the ERROR was show the screen. i don't know which step have the problem in my procedure. under is my maple file,(if you can't understand what mean of this procedure,just send me E-mail). regards View 1984_new equation with water on MapleNet or Download 1984_new equation with water
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