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\begin{aligned}&\mathrm{V}^{\prime}=\mathrm{x}(2)(8-2 \mathrm{x})(-2)+(8-2 \mathrm{x})^{2}=(8-2 \mathrm{x})(-4 \mathrm{x}+8-2 \mathrm{x})=(8-2 \mathrm{x})(8-6 \mathrm{x})=4(4-\mathrm{x})(4-3 \mathrm{x}) \text { so } \\&\mathrm{V}^{\prime} \quad \begin{cases} < 0 & \text { if } 4 / 3 < \mathrm{x} \\ > 0 & \text { if } 0 < \mathrm{x} < 4 / 3\end{cases}\end{aligned}