If \( f: \mathbb{R} \to \mathbb{R} \) is defined by
\[
f(x) =
\begin{cases}
\dfrac{2 \sin x - \sin 2x}{2x \cos x}, & \text{if } x \ne 0 \\
a, & \text{if } x = 0
\end{cases}
\]
then the value of \( a \) so that \( f \) is continuous at \( 0 \) is
Show Hint
Use L'Hopital's Rule for $0/0$ or $\infty/\infty$ limit forms.