give me Nan for r.

I have to answer this question which is dificult to do it by hand so I tried to search for another method and I found this code but it won't work with me, and can you explain why yhe use of syms r phir phi in the begning? and why did they use eval?

clc; clear all; close all;

syms r phir phi

tol = 1e-6; %setting tolerance%

dr = tol + 1;

g = 0.9;

Ms = 100; %value of Ms was given%

Ma = 0.1; %value of Ma was given%

Ms_ = Ms*(1-g);

Mt_ = Ma + Ms_;

D = (1/3)* (Mt_);

del = sqrt(D/Ma); % del stand for delta%

y = 0.1 - (exp(-r/del)/(4*pi*D*r)); % the function I have from calculation%

r = 1;%assumption that r=1%

while (abs(dr) > tol)

dr = eval(y/(diff(y))); % to calculate dr%

r = r - dr; % the new value of r%

end

fprintf('R = '); %printing the result%

r

Answer 1

[the cyclist]

The syms command is because that code is using the Symbolic Math Toolbox.