Re: [微積] 關於極限
※ 引述《simonsimon00 (simon)》之銘言:
: Given that
: lim f(x)=0 lim p(x)=∞
: x→a x→a
: Which of the following limits are inderminate forms? For those that
: are not an indeterminate form, evaluate the limit where possible.
: p(x)
: lim ----
: x→a f(x)
: Answer : ∞ , -∞ , or does not exist
: 我不知道如何解這道題目 麻煩各位幫我解惑 謝謝
1. ∞ 例如 a = 0, f(x) = |x| , p(x) = 1/|x|
2. -∞ 例如 a = 0, f(x) = -|x| , p(x) = 1/|x|
3. 極限不存在 例如 a = 0, f(x) = x , p(x) = 1/x^2
p(x)
4. 不會有 lim ------ = L,-∞ < L < ∞ 這種情況出現
x→a f(x)
(i) -∞ < a < ∞
存在 δ > 0 使得當 0 < |x-a| < δ
p(x)
L - 1 < ------ < L + 1
f(x)
|p(x)| ≦ ( |L| + 1 ) |f(x)|
因此 lim p(x) = 0, 產生矛盾
x→a
(ii) a = ∞
存在 K > 0 使得當 x > K 時
p(x)
L - 1 < ------- < L + 1
f(x)
剩下的類似作法
(iii) a = -∞
同 (ii)
大概這樣吧。
--
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