Question
$\mathop {\lim }\limits_{x \to 0} \left[ {\frac{{\sin (x + a) + \sin (a - x) - 2\sin a}}{{x\sin x}}} \right] = $
$ = - 2\,\sin a\,.\,\frac{{(1 - \cos x)}}{{{x^2}}}\,.\,\left( {\frac{x}{{\sin x}}} \right)$
$ = \mathop {\lim }\limits_{x \to 0} \,\, - 2\sin a\,.\,\frac{{2\,{{\sin }^2}(x/2)}}{{4\,{{\left( {\frac{x}{2}} \right)}^2}\,\left( {\frac{{\sin x}}{x}} \right)}} = - \sin a$.
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