Tom K. answered 12/11/20
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As sin(x) = 1/2i(eix-e-ix), sin3(x) = (1/2i(eix-e-ix))3 =
-1/8i(e3ix -3eix + 3e-ix -e-3ix) =
-1/4(1/2i(e3ix -e-3ix)) + 3/4(1/2i(eix-e-ix)) =
-1/4 sin(3x) + 3/4 sin(x)