Newton's Method to Solve Equation

I trying to write a code to solve an equation using newton's method and given three inputs.

So far I have

syms L h T g
f_L = L-g*T*T/(2*pi)*tanh(2*pi*h/L)
Df = diff(f_L,L)

The variable Df is in symbolic form but I need to convert it to numeric. Any ideas how to do this?

Thank you

Answer 1

[Yongjian Feng]

You need to substitute symbols with values, right?

L = 1.0;
h = 2.0;
T = 3.0;
g = 9.8;

% substitute
ret = subs(Df);
% convert
res = double(ret);