something problem with diff

Hello!This is my code.

syms x;

m = 5; h = 2*pi*x; q= besseli(5,h); k1 = h.^2-q; k = [m*q.*(h-3).*5 0 ; 0 q.*h-2 ]

a = det(k) pretty(diff(k(x)),x) Then,I got this message:

Invalid indexing or function definition. When defining a function, ensure that the body of the function is a SYM object. When indexing, the input must be numeric, logical or ':'.

What does it mean?

Thanks!

Answer 1

[Star Strider]

You haven’t declared k as a function of x previously, so the Symbolic Math Toolbox assumes x is a subscript (that actually makes sense considering that k is a matrix).

Do this instead:

syms x;

m = 5; 
h = 2*pi*x; 
q= besseli(5,h); 
k1 = h.^2-q; 
k = [m*q.*(h-3).*5 0 ; 0 q.*h-2 ]

a = det(k) 

fprintf('\n\ndiff(k,x) = \n\n')         % This makes the output easier to read
pretty(diff(k,x))                       % <== Changed line

produces:

diff(k,x) = 

/ pi besseli(5, 2 pi x) 50 + #1 (2 pi x - 3) 25,                  0                  \
|                                                                                    |
\                       0,                       pi besseli(5, 2 pi x) 2 + pi x #1 2 /

where

                                     besseli(5, 2 pi x) 5
     #1 == pi besseli(4, 2 pi x) 2 - --------------------
                                               x