mac lab2 bis impl

function xc = bisect(f, a, b, tol)
    fa=f(a);
    fb=f(b);
    if sign(fa)*sign(fb) >= 0
        error('f(a)f(b) not satisfied!')
    end
    while (b - a) / 2 > tol
        c = (a+b)/2;
        fc = f(c);
        if fc == 0
            break
        end
        if sign(fc) * sign(fa) < 0
            b = c;
        else
            a = c;
            fa = fc;
        end
    end
    xc = (a + b) / 2;
end