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\begin{aligned}&T=\frac{x}{8}+\frac{1}{2} \sqrt{40^{2}+(60-x)^{2}} \\&T^{\prime}=\frac{1}{8}+\frac{1}{2} \frac{1}{2}\left(40^{2}+(60-x)^{2}\right)^{-1 / 2} \cdot 2 \cdot(60-x) \cdot(-1)=\frac{1}{8}-\frac{60-x}{2 \sqrt{40^{2}+(60-x)^{2}}}\end{aligned}