Answer
Kindly see the explanation below.
Work Step by Step
Let us consider $R_n$ be the approximation obtained by using $n$ rectangles of same width.
The exact area can be found under the graph of $f$:
$\bf{A=\lim\limits_{n\to \infty} f(x_k)\triangle x}$
where $\triangle x=\dfrac{b-a}{n}$
