limit

2012-04-24 8:44 pm
f(x) is a function such that

圖片參考:http://imgcld.yimg.com/8/n/HA04937267/o/701204240023513873398630.jpg

evaluate the following limits.(唔洗計,我有ans)
(a)lim x->0 [xf(2x)]
(b)lim x->0 [(x^3)f(x^2)]
問題:
上面幅圖到
lim x->0 [xf(x)] (x是independent variable)
= (lim x->0 x)[lim x->0 f(x)]
=0
點解題目會寫lim x->0 [xf(x)]=1?
有無example可以提供?
定係我計錯?
更新1:

謝謝回答 其實我計到a同b 我只是不懂為甚麼 lim x->0 [xf(x)] =/= (lim x->0 x)[lim x->0 f(x)] 因為書中曾提及其中一個property lim x->a [f(x)g(x)] = [lim x->a f(x)][lim x->a g(x)] 套用在這的話 就是 lim x->0 [xf(x)] = (lim x->0 x)[lim x->0 f(x)] 有甚麼地方錯了?

回答 (3)

2012-04-24 9:38 pm
✔ 最佳答案
The question is easy to slove!


圖片參考:http://imgcld.yimg.com/8/n/AE03715826/o/701204240023513873398640.jpg


2012-04-24 16:30:15 補充:
Well,

lim f(x)=m and lim g(x)=n then
x→a x→a

lim f(x)g(x)=lim f(x)∙ lim g(x) =mn
x→a x→a x→a
The converse is not true!

2012-04-25 07:54:24 補充:
極限性質說
lim f(x)=m 且
x→a
lim g(x)=n ,則
x→a
lim f(x)g(x)
x→a
=(lim x→a f(x))(lim x→a g(x))
反之不一定正確!
所以已知lim x->0 [xf(x)]=1
不可拆成
lim x->0 [xf(x)] = (lim x->0 x)[lim x->0 f(x)]
2012-04-26 11:02 pm
f(x) = 1/x 不就行了?
2012-04-24 10:09 pm
Property的網址
http://tutorial.math.lamar.edu/Classes/CalcI/LimitProofs.aspx#Extras_Limit_LimitProp
謝謝

2012-04-24 17:31:24 補充:
對不起 我不明白= =
你可以用中文解釋嗎?感謝

2012-04-25 23:52:47 補充:
明白~謝謝
有沒有一些例子提供~?感謝大大


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