(b)
Required area of shaded portion $OABCO$
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$= \int_{0}^{4}{\left( \sqrt{4x} - \frac{x^{2}}{4} \right)dx}$
${= \left\lbrack \frac{{2x}^{3/2}}{3/2} - \frac{x^{3}}{12} \right\rbrack}_{0}^{4}$
$= \left\lbrack \frac{32}{3} - \frac{16}{3} \right\rbrack$
$= \frac{16}{3}sq\ units$