If you want to evaluate dC(x)/dx, you have to have the Symbolic Math Toolbox:
syms x
a = 0.1;
b = 0.3;
l = 0.25;
e = 71E-11;
C(x) = (2*pi*e*l)./acosh((a.^2 + b.^2 - x.^2)./(2*a*b));
dC = diff(C,x);
x = 0.05;
dCx = vpa(dC(x), 5)
dCx =
1.2753e-9
The vpa function limits the number of digits the numerical result is displayed with, and converts fractions that might otherwise result from the derivation to decimals.
The Symbolic Math Toolbox is an add-on (not part of core MATLAB), but worth buying if you do relatively frequent symbolic calculations.
