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Find the average value of $f( x )=\cfrac{\sin x}{x}$ on the interval $ \left[ \cfrac {\pi}{2}, \cfrac {3\pi}{2} \right]$ using a Left Riemann Sum with $2$ equal subintervals.

$0$

$-1$

$-\cfrac {1}{\pi}$

$\cfrac {1}{\pi}$