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Answers for “为什么视频要包含 $e^{2x}$ 和 $e^x$ 的例子?”

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Understand why

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The example is included to demonstrate that the domination hypothesis alone is insufficient if the dominating function's integral diverges. It serves as a warning that if the finite-integral condition is not met, the desired interchange of integral and sum may fail.

Conditions: The example is presented as a warning illustration on the interval [0,∞)[0,\infty).; The function e2xe^{2x} dominates exe^x, but ∫0∞e2x dx\int_0^{\infty} e^{2x}\,dx diverges.

Meet the concept

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The three hypotheses are: (1) a sequence of functions fnf_n converges to a function ff on the interval [a,b][a,b]; (2) there exists a dominating function gg such that ∣g(x)∣≥∣fn(x)∣|g(x)| \ge |f_n(x)| for all nn and all x∈[a,b]x \in [a,b]; and (3) the integral of the dominating function over the interval is finite, meaning ∫abg(x) dx<∞\int_a^b g(x)\,dx < \infty.

Conditions: The theorem is presented in a calculus-oriented form rather than a full measure-theoretic proof.; The interval [a,b][a,b] is a closed, bounded real interval.