If f(x)=sinx, then
f(2x)=sin2x=2sinxcosx (double angle formula).
Let's consider the right side of the equality:
2f(x)f(2π−x)=2sinxsin(2π−x)=2sinxcosx,
because sin(2π−x)=cosx (co-function identity).
So we have:
f(2x)=2sinxcosx=2f(x)f(2π−x),
and the statement is proved.
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