how get second derivative of a function with one variable? I keep getting not enough input arguments?

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Ali Mir
Ali Mir 2023-2-23
评论： Walter Roberson 2023-2-23
Can someone help with this error? it seems I did everything according to tutorials
I was asked to use MATLAB function to calculate second derivative. I created the function but I cannot export the secnd derivative from it. i also tried to give it some value and it works fine. Just diff() does not work

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回答（1 个）

Stephen23
Stephen23 2023-2-23
编辑：Stephen23 2023-2-23
Ran in:
"it seems I did everything according to tutorials"
Sort of: you defined a function that works on numeric data, but then you have attempted to use this with symbolic differentiation (i.e. DIFF from the symbolic toolbox). That will not work. If you want to perform symbolic differentiation, then you will need to define a symbolic formula. See the examples here:
https://www.mathworks.com/help/symbolic/differentiation.html
syms t
f = exp(5*t) + t^2
f = 
d = diff(f,t) % default = 1st derivative
d = 
d = diff(f,t,2) % 2nd derivative
d = 
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Walter Roberson
Walter Roberson 2023-2-23
Ran in:
What those tell you is that you cannot use diff to find the derivative of a MATLAB function -- not unless you pass a symbolic variable into the function so that diff() is operating on a symbolic expression.
You can use diff() to find the derivative of an anonymous function or of a symbolic function
syms t
f1 = @(t) exp(5*t) + t^2
f1 = function_handle with value:
@(t)exp(5*t)+t^2
diff(f1, t, t)
ans = 
f2(t) = exp(5*t) + t^2
f2(t) = 
diff(f2, t, t)
ans(t) = 

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