% 由于函数单调，因此可以用二分法求 x/log(9x) = k 的解

function result = solve_test(k)
    eps = 1e-3;
    l = 1;
    r = k;
    while r - l > eps
        mid = (l+r) / 2;
        if (foo(mid) < foo(r))
            l = mid;
        else
            r = mid;
        end
    end
    result = l;
end

function f = foo(x)
    f = x / log(9 * x);
end