Hi, I am having a hard time finding the root to a polynomial function using the bisection method

Hi,

I am having a hard time finding the root to a polynomial function using the bisection method with the precision of 0.001. This is not a homework question. I am new at this and just trying to learn. This is what I have, but I have errors. I do appreciate any help or advice.

function  root[x_lower,x_upper] 3*x.^3-10;
x_lower=-5;
x_upper=5;
x_mid= (x_lower* X_upper)/2;
while abs (f(x_mid))>0.001
    if (f(x_mid)*f(x_upper))<0
        x_lower=x_mid
    else
        x_upper=x_mid
    end
    x_mid=(x_lower+x_upper)/2;
end
fprintf('the root is %g\n', x_mid)

[Naeem Khan]

what is this last command used for "fprintf('the root is %g\n', x_mid)"

[Walter Roberson]

The fprintf() in that form displays to the command window the text

the root is

followed on the same line by the value of the variable x_mid, with the value represented in direct decimal notation if its absolute value is in the range 0.0001 to 999999, and otherwise representing it in scientific notation. After that, newline is output to move on to the next line

Answer 1

[Walter Roberson]

Change

function  root[x_lower,x_upper] 3*x.^3-10;

to

function  root(x_lower,x_upper)
  f = @(x) 3*x.^3-10;

[Nasser Yari]

Thank you, it helped clear up some confusion, but I get the following error: Undefined function or variable 'X_upper'.

Error in root (line 5) x_mid= (x_lower+ X_upper)/2;

[Walter Roberson]

X_upper and x_upper are different variable names. You have used both. You need to be consistent.

[Nasser Yari]

Walter, you're the best. Thanks it works.