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为揭露elim吃屎成痴不识自然数的危害,现在我们根据Weierstrass 极限定义直接证明\(\displaystyle\lim_{n \to \infty}n\)、\(\displaystyle\lim_{n \to \infty}2n\)、\(\displaystyle\lim_{n \to \infty}2^n\)、\(\displaystyle\lim_{n \to \infty}10^n\)、……\(\in\mathbb{N}\)!
〖证明:〗根据Weierstrass极限定义:\(\displaystyle\lim_{n \to \infty}x_n=a\)\(对\forall \varepsilon>0\iff \exists\)正整数\(N_\varepsilon\)\((=[\tfrac{1}{\varepsilon}]+1)\),当\(n>N_{\varepsilon}\),有\(|x_n-a|<{\varepsilon}\)( Weierstrass 极限定义的符号表示式参见同济大学《高等数学》第七版 上册P21页第24行),特别的,令\(\varepsilon=(\displaystyle\lim_{n \to \infty}n)^{-1}\),则\(N_\varepsilon\)\((=\displaystyle\lim_{n \to \infty}n\))=\(\displaystyle\lim_{n \to \infty}n\)\(\in\mathbb{N}\)
同理:
令\(\varepsilon=(\displaystyle\lim_{n \to \infty}2n)^{-1}\),则\(N_\varepsilon\)\((=\displaystyle\lim_{n \to \infty}2n\))=\(\displaystyle\lim_{n \to \infty}2n\)\(\in\mathbb{N}\);
令\(\varepsilon=(\displaystyle\lim_{n \to \infty}2^n)^{-1}\),则\(N_\varepsilon\)\((=\displaystyle\lim_{n \to \infty}2^n\))=\(\displaystyle\lim_{n \to \infty}2^n\)\(\in\mathbb{N}\);
令\(\varepsilon=(\displaystyle\lim_{n \to \infty}10^n)^{-1}\),则\(N_\varepsilon\)\((=\displaystyle\lim_{n \to \infty}10^n\))=\(\displaystyle\lim_{n \to \infty}10^n\)\(\in\mathbb{N}\);
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