Question
$\sin 12^\circ \sin 48^\circ \sin 54^\circ = $
$ = \frac{1}{2}\,\left[ {\frac{{\sqrt 5 + 1}}{4} - \frac{1}{2}} \right]\,\left[ {\frac{{\sqrt 5 + 1}}{4}} \right] $
$= \frac{1}{2}\,\left[ {\frac{{\sqrt 5 - 1}}{4}} \right]\,\left[ {\frac{{\sqrt 5 + 1}}{4}} \right]$
$ = \frac{{5 - 1}}{{32}} = \frac{4}{{32}} = \frac{1}{8}$.
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