Proving a second order special limit without derivatives
$begingroup$
The special limit
$$
lim_{x to 0} frac{e^x-x-1}{x^2}=frac 1 2
$$
can be proved by Taylor expansion or with L'Hôpital's rule. Is it possoble to prove it without using derivatives?
real-analysis limits limits-without-lhopital
$endgroup$
add a comment |
$begingroup$
The special limit
$$
lim_{x to 0} frac{e^x-x-1}{x^2}=frac 1 2
$$
can be proved by Taylor expansion or with L'Hôpital's rule. Is it possoble to prove it without using derivatives?
real-analysis limits limits-without-lhopital
$endgroup$
2
$begingroup$
Is there a reason that neither of these methods can be used?
$endgroup$
– Henry Lee
Dec 2 '18 at 18:54
1
$begingroup$
If we define $e^x$ as $sum_{ngeq 0}frac{x^n}{n!}$ that is trivial.
$endgroup$
– Jack D'Aurizio
Dec 2 '18 at 19:07
$begingroup$
See math.stackexchange.com/questions/387333/…
$endgroup$
– lab bhattacharjee
Dec 5 '18 at 8:45
add a comment |
$begingroup$
The special limit
$$
lim_{x to 0} frac{e^x-x-1}{x^2}=frac 1 2
$$
can be proved by Taylor expansion or with L'Hôpital's rule. Is it possoble to prove it without using derivatives?
real-analysis limits limits-without-lhopital
$endgroup$
The special limit
$$
lim_{x to 0} frac{e^x-x-1}{x^2}=frac 1 2
$$
can be proved by Taylor expansion or with L'Hôpital's rule. Is it possoble to prove it without using derivatives?
real-analysis limits limits-without-lhopital
real-analysis limits limits-without-lhopital
edited Dec 2 '18 at 19:38
gimusi
92.9k94494
92.9k94494
asked Dec 2 '18 at 18:52
Marco DisceMarco Disce
1,3961216
1,3961216
2
$begingroup$
Is there a reason that neither of these methods can be used?
$endgroup$
– Henry Lee
Dec 2 '18 at 18:54
1
$begingroup$
If we define $e^x$ as $sum_{ngeq 0}frac{x^n}{n!}$ that is trivial.
$endgroup$
– Jack D'Aurizio
Dec 2 '18 at 19:07
$begingroup$
See math.stackexchange.com/questions/387333/…
$endgroup$
– lab bhattacharjee
Dec 5 '18 at 8:45
add a comment |
2
$begingroup$
Is there a reason that neither of these methods can be used?
$endgroup$
– Henry Lee
Dec 2 '18 at 18:54
1
$begingroup$
If we define $e^x$ as $sum_{ngeq 0}frac{x^n}{n!}$ that is trivial.
$endgroup$
– Jack D'Aurizio
Dec 2 '18 at 19:07
$begingroup$
See math.stackexchange.com/questions/387333/…
$endgroup$
– lab bhattacharjee
Dec 5 '18 at 8:45
2
2
$begingroup$
Is there a reason that neither of these methods can be used?
$endgroup$
– Henry Lee
Dec 2 '18 at 18:54
$begingroup$
Is there a reason that neither of these methods can be used?
$endgroup$
– Henry Lee
Dec 2 '18 at 18:54
1
1
$begingroup$
If we define $e^x$ as $sum_{ngeq 0}frac{x^n}{n!}$ that is trivial.
$endgroup$
– Jack D'Aurizio
Dec 2 '18 at 19:07
$begingroup$
If we define $e^x$ as $sum_{ngeq 0}frac{x^n}{n!}$ that is trivial.
$endgroup$
– Jack D'Aurizio
Dec 2 '18 at 19:07
$begingroup$
See math.stackexchange.com/questions/387333/…
$endgroup$
– lab bhattacharjee
Dec 5 '18 at 8:45
$begingroup$
See math.stackexchange.com/questions/387333/…
$endgroup$
– lab bhattacharjee
Dec 5 '18 at 8:45
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
$$frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y)e^{xy},dy $$
hence by the dominated convergence theorem
$$ lim_{xto 0}frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y),dy = frac{1}{2}.$$
$endgroup$
add a comment |
$begingroup$
We have by $x=2y$
$$frac{e^x-x-1}{x^2}=frac{e^{2y}-2y-1}{4y^2}=frac{(e^y-1)^2+2e^y-2y-2}{4y^2}=frac14left(frac{e^y-1}{y}right)^2+frac12frac{e^y-y-1}{y^2}$$
therefore assuming that the limit exists we have
$$L=frac14+frac12L implies L=frac12$$
Refer also to
- Are all limits solvable without L'Hôpital Rule or Series Expansion
$endgroup$
$begingroup$
Since L'Hôpital and Taylor expansion are explicitly ruled out, you might want to mention how $(e^y-1)/y to 1$ is obtained.
$endgroup$
– Martin R
Dec 2 '18 at 21:25
$begingroup$
@MartinR My interpretation is that we can't use l'Hopital and Taylor to solve the limit but I think we can use the well known standard limit which, as you can know I suppose, can be derived without Hopital and Taylor. I'll add a reference to that anyway. Thanks
$endgroup$
– gimusi
Dec 2 '18 at 21:38
add a comment |
$begingroup$
Let $$f(x)=e^{sqrt{x}}-sqrt{x}$$
Then your limit is
$$lim_{Xto0^+}frac{f(X)-f(0)}{X-0}$$
$endgroup$
$begingroup$
In the limit expression, you mean $f(x^2)$.
$endgroup$
– Anurag A
Dec 2 '18 at 19:01
$begingroup$
@AnuragA No. {}{}{}{}{}{}.
$endgroup$
– hamam_Abdallah
Dec 2 '18 at 19:01
$begingroup$
is that by derivative?
$endgroup$
– gimusi
Dec 2 '18 at 20:06
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3023041%2fproving-a-second-order-special-limit-without-derivatives%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
$$frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y)e^{xy},dy $$
hence by the dominated convergence theorem
$$ lim_{xto 0}frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y),dy = frac{1}{2}.$$
$endgroup$
add a comment |
$begingroup$
$$frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y)e^{xy},dy $$
hence by the dominated convergence theorem
$$ lim_{xto 0}frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y),dy = frac{1}{2}.$$
$endgroup$
add a comment |
$begingroup$
$$frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y)e^{xy},dy $$
hence by the dominated convergence theorem
$$ lim_{xto 0}frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y),dy = frac{1}{2}.$$
$endgroup$
$$frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y)e^{xy},dy $$
hence by the dominated convergence theorem
$$ lim_{xto 0}frac{e^x-1-x}{x^2}=int_{0}^{1}(1-y),dy = frac{1}{2}.$$
answered Dec 2 '18 at 19:10
Jack D'AurizioJack D'Aurizio
288k33280660
288k33280660
add a comment |
add a comment |
$begingroup$
We have by $x=2y$
$$frac{e^x-x-1}{x^2}=frac{e^{2y}-2y-1}{4y^2}=frac{(e^y-1)^2+2e^y-2y-2}{4y^2}=frac14left(frac{e^y-1}{y}right)^2+frac12frac{e^y-y-1}{y^2}$$
therefore assuming that the limit exists we have
$$L=frac14+frac12L implies L=frac12$$
Refer also to
- Are all limits solvable without L'Hôpital Rule or Series Expansion
$endgroup$
$begingroup$
Since L'Hôpital and Taylor expansion are explicitly ruled out, you might want to mention how $(e^y-1)/y to 1$ is obtained.
$endgroup$
– Martin R
Dec 2 '18 at 21:25
$begingroup$
@MartinR My interpretation is that we can't use l'Hopital and Taylor to solve the limit but I think we can use the well known standard limit which, as you can know I suppose, can be derived without Hopital and Taylor. I'll add a reference to that anyway. Thanks
$endgroup$
– gimusi
Dec 2 '18 at 21:38
add a comment |
$begingroup$
We have by $x=2y$
$$frac{e^x-x-1}{x^2}=frac{e^{2y}-2y-1}{4y^2}=frac{(e^y-1)^2+2e^y-2y-2}{4y^2}=frac14left(frac{e^y-1}{y}right)^2+frac12frac{e^y-y-1}{y^2}$$
therefore assuming that the limit exists we have
$$L=frac14+frac12L implies L=frac12$$
Refer also to
- Are all limits solvable without L'Hôpital Rule or Series Expansion
$endgroup$
$begingroup$
Since L'Hôpital and Taylor expansion are explicitly ruled out, you might want to mention how $(e^y-1)/y to 1$ is obtained.
$endgroup$
– Martin R
Dec 2 '18 at 21:25
$begingroup$
@MartinR My interpretation is that we can't use l'Hopital and Taylor to solve the limit but I think we can use the well known standard limit which, as you can know I suppose, can be derived without Hopital and Taylor. I'll add a reference to that anyway. Thanks
$endgroup$
– gimusi
Dec 2 '18 at 21:38
add a comment |
$begingroup$
We have by $x=2y$
$$frac{e^x-x-1}{x^2}=frac{e^{2y}-2y-1}{4y^2}=frac{(e^y-1)^2+2e^y-2y-2}{4y^2}=frac14left(frac{e^y-1}{y}right)^2+frac12frac{e^y-y-1}{y^2}$$
therefore assuming that the limit exists we have
$$L=frac14+frac12L implies L=frac12$$
Refer also to
- Are all limits solvable without L'Hôpital Rule or Series Expansion
$endgroup$
We have by $x=2y$
$$frac{e^x-x-1}{x^2}=frac{e^{2y}-2y-1}{4y^2}=frac{(e^y-1)^2+2e^y-2y-2}{4y^2}=frac14left(frac{e^y-1}{y}right)^2+frac12frac{e^y-y-1}{y^2}$$
therefore assuming that the limit exists we have
$$L=frac14+frac12L implies L=frac12$$
Refer also to
- Are all limits solvable without L'Hôpital Rule or Series Expansion
answered Dec 2 '18 at 19:11
gimusigimusi
92.9k94494
92.9k94494
$begingroup$
Since L'Hôpital and Taylor expansion are explicitly ruled out, you might want to mention how $(e^y-1)/y to 1$ is obtained.
$endgroup$
– Martin R
Dec 2 '18 at 21:25
$begingroup$
@MartinR My interpretation is that we can't use l'Hopital and Taylor to solve the limit but I think we can use the well known standard limit which, as you can know I suppose, can be derived without Hopital and Taylor. I'll add a reference to that anyway. Thanks
$endgroup$
– gimusi
Dec 2 '18 at 21:38
add a comment |
$begingroup$
Since L'Hôpital and Taylor expansion are explicitly ruled out, you might want to mention how $(e^y-1)/y to 1$ is obtained.
$endgroup$
– Martin R
Dec 2 '18 at 21:25
$begingroup$
@MartinR My interpretation is that we can't use l'Hopital and Taylor to solve the limit but I think we can use the well known standard limit which, as you can know I suppose, can be derived without Hopital and Taylor. I'll add a reference to that anyway. Thanks
$endgroup$
– gimusi
Dec 2 '18 at 21:38
$begingroup$
Since L'Hôpital and Taylor expansion are explicitly ruled out, you might want to mention how $(e^y-1)/y to 1$ is obtained.
$endgroup$
– Martin R
Dec 2 '18 at 21:25
$begingroup$
Since L'Hôpital and Taylor expansion are explicitly ruled out, you might want to mention how $(e^y-1)/y to 1$ is obtained.
$endgroup$
– Martin R
Dec 2 '18 at 21:25
$begingroup$
@MartinR My interpretation is that we can't use l'Hopital and Taylor to solve the limit but I think we can use the well known standard limit which, as you can know I suppose, can be derived without Hopital and Taylor. I'll add a reference to that anyway. Thanks
$endgroup$
– gimusi
Dec 2 '18 at 21:38
$begingroup$
@MartinR My interpretation is that we can't use l'Hopital and Taylor to solve the limit but I think we can use the well known standard limit which, as you can know I suppose, can be derived without Hopital and Taylor. I'll add a reference to that anyway. Thanks
$endgroup$
– gimusi
Dec 2 '18 at 21:38
add a comment |
$begingroup$
Let $$f(x)=e^{sqrt{x}}-sqrt{x}$$
Then your limit is
$$lim_{Xto0^+}frac{f(X)-f(0)}{X-0}$$
$endgroup$
$begingroup$
In the limit expression, you mean $f(x^2)$.
$endgroup$
– Anurag A
Dec 2 '18 at 19:01
$begingroup$
@AnuragA No. {}{}{}{}{}{}.
$endgroup$
– hamam_Abdallah
Dec 2 '18 at 19:01
$begingroup$
is that by derivative?
$endgroup$
– gimusi
Dec 2 '18 at 20:06
add a comment |
$begingroup$
Let $$f(x)=e^{sqrt{x}}-sqrt{x}$$
Then your limit is
$$lim_{Xto0^+}frac{f(X)-f(0)}{X-0}$$
$endgroup$
$begingroup$
In the limit expression, you mean $f(x^2)$.
$endgroup$
– Anurag A
Dec 2 '18 at 19:01
$begingroup$
@AnuragA No. {}{}{}{}{}{}.
$endgroup$
– hamam_Abdallah
Dec 2 '18 at 19:01
$begingroup$
is that by derivative?
$endgroup$
– gimusi
Dec 2 '18 at 20:06
add a comment |
$begingroup$
Let $$f(x)=e^{sqrt{x}}-sqrt{x}$$
Then your limit is
$$lim_{Xto0^+}frac{f(X)-f(0)}{X-0}$$
$endgroup$
Let $$f(x)=e^{sqrt{x}}-sqrt{x}$$
Then your limit is
$$lim_{Xto0^+}frac{f(X)-f(0)}{X-0}$$
answered Dec 2 '18 at 18:59
hamam_Abdallahhamam_Abdallah
38k21634
38k21634
$begingroup$
In the limit expression, you mean $f(x^2)$.
$endgroup$
– Anurag A
Dec 2 '18 at 19:01
$begingroup$
@AnuragA No. {}{}{}{}{}{}.
$endgroup$
– hamam_Abdallah
Dec 2 '18 at 19:01
$begingroup$
is that by derivative?
$endgroup$
– gimusi
Dec 2 '18 at 20:06
add a comment |
$begingroup$
In the limit expression, you mean $f(x^2)$.
$endgroup$
– Anurag A
Dec 2 '18 at 19:01
$begingroup$
@AnuragA No. {}{}{}{}{}{}.
$endgroup$
– hamam_Abdallah
Dec 2 '18 at 19:01
$begingroup$
is that by derivative?
$endgroup$
– gimusi
Dec 2 '18 at 20:06
$begingroup$
In the limit expression, you mean $f(x^2)$.
$endgroup$
– Anurag A
Dec 2 '18 at 19:01
$begingroup$
In the limit expression, you mean $f(x^2)$.
$endgroup$
– Anurag A
Dec 2 '18 at 19:01
$begingroup$
@AnuragA No. {}{}{}{}{}{}.
$endgroup$
– hamam_Abdallah
Dec 2 '18 at 19:01
$begingroup$
@AnuragA No. {}{}{}{}{}{}.
$endgroup$
– hamam_Abdallah
Dec 2 '18 at 19:01
$begingroup$
is that by derivative?
$endgroup$
– gimusi
Dec 2 '18 at 20:06
$begingroup$
is that by derivative?
$endgroup$
– gimusi
Dec 2 '18 at 20:06
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3023041%2fproving-a-second-order-special-limit-without-derivatives%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
2
$begingroup$
Is there a reason that neither of these methods can be used?
$endgroup$
– Henry Lee
Dec 2 '18 at 18:54
1
$begingroup$
If we define $e^x$ as $sum_{ngeq 0}frac{x^n}{n!}$ that is trivial.
$endgroup$
– Jack D'Aurizio
Dec 2 '18 at 19:07
$begingroup$
See math.stackexchange.com/questions/387333/…
$endgroup$
– lab bhattacharjee
Dec 5 '18 at 8:45