NonConstants
 Usage
• NonConstants是D的可选项，用来给出一个明确依赖微分变量的对象构成的列表.
Notes
• 如果c不在NonConstants列表中出现，且c和x不是同一表达式，则D[c, x]为0.
 Further Examples
Normally, if you differentiate a function with respect to x, say, Mathematica will treat all other parameters as constants.
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By specifying that t depends upon x, you can get the desired result for such expressions.
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