MATLAB: Need help with this bisection method code!

algorithmbisectionbisection method

Im studying for a math test and on a old test there is a task about bisection. The task is to solve x^2=2 with the bisection method and the precision should be with 10 decimals. The problem is that it seems like the teachers recommended solution to the task isn't quite right. Here's the code:
a=1;
b=2;
tol=1e-10;
fa=a*a-2;
fb=b*b-2;
while b-a>tol
    c=(a+b)/2;
    fc=c+c-2;
    if fa*fc<0
        b=c;
        fb=fc;
    elseif fb*fc<0
        a=c;
        fa=fc;
    else
        break
    end
end
x=c
I keep getting the answer x= 1.000000000087311
What am I doing wrong, have been stuck with this problem for a while now so I might have missed some obvious misstakes
/Johan

Best Answer

Johan - for the while condition, you probably want the absolute value of the difference to handle the case where the roots could be negative. So just change this line to
 while abs(b-a) > tol
Also, look at how you are calculating fc
 fc = c+c-2;
This should probably be
 fx = c*c-2;
instead, since you are trying to find the root of x^2-2. Try making these two changes and see if that helps find the correct solution.
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