不知道如何證明, 只知道 y=e^kx 滿足, 麻煩各位大大提供解決方法！ 謝謝囉！
Show that, if f: R->R is any differentiable, nonconstant function such
that f(x+y)=f(x)f(y), then there exists a constant c such that, for all
x belongs to R f'(x) = cf(x)
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