How to input new value as old value using while loop to compute relative error?

I need help on how can I input new value as an old value

Whenever I run the code, the r_e (relative error) equation goes up and gives me a wrong answer

How do I fix this?

f_x= sin (5x) + cos (2x)
for i=1:n

f_l = f_x(x_l);
f_u = f_x(x_u);
p = (x_l+x_u)/2;
f_m = f_x(p);

  if (f_l*f_m)<0
    x_u=p;
  elseif (f_u*f_m)<0
    x_l=p;
  end
    p_old = p;
    r_e = abs(p-p_old/(p));
    table(i, :)={i-1 p r_e};
    fprintf('%d                     %d                      %d \n', table{i, :});
    
end

Answer 1

[VBBV]

r_e = abs((p-p_old)/(p));

[VBBV]

With the parenthesis for numerator and check if r_e is computed correctly now.

[VBBV]

f_x= @(x) sin (5*x) + cos (2*x)

f_x = function_handle with value:
    @(x)sin(5*x)+cos(2*x)

p_old = 0;  % give an initial value to it 
x_l =2; x_u = 10;
n = 20;
for i=1:n

f_l = f_x(x_l);
f_u = f_x(x_u);
p = (x_l+x_u)/2;
f_m = f_x(p);

  if (f_l*f_m)<0
    x_u=p;
  elseif (f_u*f_m)<0
    x_l=p;
  end
    
    r_e = abs((p-p_old)/(p));  
    p_old = p;               %  assign the p_old after relative error 
    table(i, :)={i-1 p r_e};
    fprintf('%d                     %d                      %d \n', table{i, :});
    
end

0                     6                      1 
1                     8                      2.500000e-01 
2                     9                      1.111111e-01 
3                     8.500000e+00                      5.882353e-02 
4                     8.750000e+00                      2.857143e-02 
5                     8.875000e+00                      1.408451e-02 
6                     8.812500e+00                      7.092199e-03 
7                     8.781250e+00                      3.558719e-03 
8                     8.765625e+00                      1.782531e-03 
9                     8.757812e+00                      8.920607e-04 
10                     8.753906e+00                      4.462294e-04 
11                     8.751953e+00                      2.231645e-04 
12                     8.750977e+00                      1.115947e-04 
13                     8.751465e+00                      5.579423e-05 
14                     8.751709e+00                      2.789634e-05 
15                     8.751587e+00                      1.394836e-05 
16                     8.751526e+00                      6.974230e-06 
17                     8.751556e+00                      3.487103e-06 
18                     8.751572e+00                      1.743548e-06 
19                     8.751579e+00                      8.717735e-07 

give an initial value to p_old and assign the value to it after relative error inside the for loop

[Ja]

Thanks a lot !