[tex]\bf u=\sqrt[3]{t^2}-3\sqrt{t^3}\implies u=t^{\frac{2}{3}}-3t^{\frac{3}{2}}\implies \cfrac{du}{dt}=\stackrel{power~rule}{\cfrac{2}{3}t^{\frac{2}{3}-1}~~-~~3\cdot \cfrac{3}{2}t^{\frac{3}{2}-1}}
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\cfrac{du}{dt}=\cfrac{2}{3}t^{-\frac{1}{3}}~~-~~\cfrac{9}{2}t^{\frac{1}{2}}\implies \cfrac{du}{dt}=\cfrac{2}{3t^{\frac{1}{3}}}~~-~~\cfrac{9t^{\frac{1}{2}}}{2}
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\cfrac{du}{dt}=\cfrac{2}{3\sqrt[3]{t}}-\cfrac{9\sqrt{t}}{2}[/tex]