How can i determine the analytical expression of the derived funtion "df(x)/dx" ?

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Mallouli Marwa
Mallouli Marwa 2017-1-23
评论： Star Strider 2017-1-23
How can i determine the analytical expression of the derived funtion "df(x)/dx"?
x=linspace (0:40)
L=3;
sol(1)=2;
psi=1;
f (x) = @(x) cos ( (sol(1)/L) *x)- cosh( (sol(1)/L) *x)+ psi *( sin( (sol(1)/L) *x)- sinh( (sol(1))

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Star Strider
Star Strider 2017-1-23
Use the Symbolic Math Toolbox:
syms x
L=sym(3);
sol(1)=sym(2);
psi=sym(1);
f (x) = cos ( (sol(1)/L) *x)- cosh( (sol(1)/L) *x)+ psi *( sin( (sol(1)/L) *x)- sinh( (sol(1))));
df_dx = diff(f,x)
df_dx_fcn = matlabFunction(df_dx)
df_dx(x) =
(2*cos((2*x)/3))/3 - (2*sin((2*x)/3))/3 - (2*sinh((2*x)/3))/3
df_dx_fcn = @(x) cos(x.*(2.0./3.0)).*(2.0./3.0)-sin(x.*(2.0./3.0)).*(2.0./3.0)-sinh(x.*(2.0./3.0)).*(2.0./3.0)
  2 个评论
Mallouli Marwa
Mallouli Marwa 2017-1-23
If i want to comput the derived function in a point y=40 "df_dy(40)"
I obtain this error
Error using mupadmex
Error in MuPAD command: Index exceeds matrix dimensions.
Star Strider
Star Strider 2017-1-23
I am not certain how you called the function, that is named ‘df_dx’, not ‘df_dy’. You can always rename it in the derivation and assignment, but you have to use the function name that exists. (I do not know if you used double quotes in your code. The double quotes (") are not valid MATLAB syntax.)
Depending on what you want, one of these will work:
y = 40;
SymOutput = df_dx(y)
SymOutput = vpa(df_dx(y))
NumOutput = df_dx_fcn(y)
SymOutput =
(2*cos(80/3))/3 - (2*sin(80/3))/3 - (2*sinh(80/3))/3
SymOutput =
-127076407709.60347598489525821488
NumOutput =
-127.0764e+009

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