If x is a real number, then $$\sqrt{\log_{e}{\frac{4x - x^2}{3}}}$$ is a real number if and only if
$$\sqrt{\log_{e}{\frac{4x - x^2}{3}}}$$ will be real if $$\log_e\ \frac{\ 4x-x^2}{3}\ \ge\ 0$$
$$\frac{\ 4x-x^2}{3}\ >=\ 1$$
$$\ 4x-x^2-3\ >=\ 0$$
$$\ x^2-4x+3\ =<\ 0$$
1=< x=< 3
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