\begin{tikzpicture}

\draw[->] (0,0) -- (6,0) node[right] {$x$} ;
\draw[->] (0,0) -- (0,5) node[above] {$y$} ;

\filldraw[fill=gray!30]
	(1,0) node[below] {$a$}
	-- (1,2)
	cos (5,4) node[right] {$f(x)$}
	-- (5,0) node[below] {$b$}
	-- cycle ;

\draw (3,1) node {$\displaystyle \int_a^b f(x) \, \mathrm{d}x$} ;

\end{tikzpicture}