(a) $\lim_{x \rightarrow 2}\frac{\int_{6}^{f(x)}{4\ t^{3}\ dt}}{x - 2}\lim_{x \rightarrow 2}{\frac{4\left\{ f(x) \right\}^{3}}{1}.f^{'}(x)}$ $= 4\left\{ f(2) \right\}^{3}.f^{'}(2)$ $= 4 \times (6)^{3}.\frac{1}{48}$ $= 18$
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