Question:
The area (in sq. units) of the region $\left\{(x, y) \in R^{2} \mid 4 x^{2} \leq y \leq 8 x+12\right\}$ is:
Correct Option: , 2
Solution:
Given curves are
$4 x^{2}=y$
$y=8 x+12$
From eqns. (i) and (ii),
$4 x^{2}=8 x+12$
$\Rightarrow x^{2}-x-3=0$
$\Rightarrow x^{2}-2 x-3=0$
$\Rightarrow x^{2}-3 x+x-3=0$
$\Rightarrow(x+1)(x-3)=0$
$\Rightarrow x=-1,3$
Required area bounded by curves is given by
$A=\int_{-1}^{3}\left(8 x+12-4 x^{2}\right) d x$
$A=\frac{8 x^{2}}{2}+12 x-\left.\frac{4 x^{3}}{3}\right|_{-1} ^{3}$
$=(4(9)+36-36)-\left(4-12+\frac{4}{3}\right)$
$=36+8-\frac{4}{3}=44-\frac{4}{3}=\frac{132-4}{3}=\frac{128}{3}$