Evaluating a symbolic function

问题:

I want to find cos(5). Why is this expression invalid:

syms x
f=sin(x)
disp(diff(f)(5))

The error is

Line: 3 Column: 12
Indexing with parentheses '()' must appear as the last operation of a valid indexing expression.

回答1:

Your error has nothing to do with symbolic variables.

It is caused by the statement diff(f)(5) - which is not something MATLAB syntax allows (as of R2019b). MATLAB interprets this as the user trying to access the 5th element of some intermediate result. If you want to know the actual value of the derivative of f at x=5, you would have to substitute the desired value of x (using subs) and convert this to some numeric format (such as double):

syms x
f = sin(x)
disp(double(subs(diff(f),x,5))) % substitute x and convert to double

回答2:

Y = cos(x) will simply provide the cosine value of x. Or all the x.

I think in your code: disp(diff(f(5))) should work.