%construct the input
syms x y z
f(x,y,z) = 2*y*z*sin(x) + 3*x*sin(z)*cos(y);
%and assume you are passing f into a different function so x y z are not still around
g = yadayada(f);
function g = yadayada(f)
%so we have an input symbolic function but for some reason
%we are not in the context where the variables came from.
%it turns out to be as simple as
g = gradient(f);
%or for greater certainty you could
g = gradient(f, symvar(f)); %which is what the simple call does
%or perhaps you have a reason to do
syms x y z
g = gradient(f(x, y, z), [x y z])
%for example it might be a symbolic function in three other
%variables that you want to re-label as x, y, z.
end
