Fixed-Point Iteration Numerical Method

Function for finding the x root of f(x) to make f(x) = 0, using the fixedpoint iteration open method

% Inputs: with examples
% AF = anonymous function equation: AF = @(x) 1-((20^2)./(9.81*(((3*x)+((x.^2)/2)).^3))).*(3+x);
% xi = initial guess x = xR, where xR = x root: xi = 0.5;
% ed = desired approximate relative error = |(current - previous)/current|: ed = 0.01;
% Outputs
% xR = x root
% err = approximate relative error
% n = number of iterations
% xRV = x root vector
% errV = approximate relative error vector
% AFD1 = anonymous function 1st derivative
% AFD2 = anonymous function 2nd derivative