Why is $\int_{0}^{\infty} f(t)x^tdt$ the continuous version of the power series $\sum_{0}^{\infty} f(n)x^n$?

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I am having trouble understanding why $\int_{0}^{\infty} f(t)x^tdt$ is the continuous analogue of the power series $\sum_{0}^{\infty} f(n)x^n$.

as in my mind a continuous power series is the following

(1) $f(0)x^0 + f(dt)x^{dt} + f(2dt)x^{2dt} + .... $

not the integral $\int_{0}^{\infty} f(t)x^tdt$

(2) $dt(f(0)x^0 + f(dt)x^{dt} + f(2dt)x^{2dt} + ....) $

as it seems to just be a continuous power series multiplied by $dt$

So why is (2) a "continuous analogue of a power series" but (1) isn't ?