Proof that $e^{ab} = left(e^aright)^b$, using the series for $e^x$
$begingroup$
I want to prove that
$$e^{ab}=left(e^{a}right)^{b}$$where
$$a,b in mathbb{R}$$ using the infinite series for $$e^{x}$$
My attempt:
$$e^{ab}=sum_{n=0}^{infty}frac{(ab)^{n}}{n!}=sum_{n=0}^{infty}frac{(a)^{n}(b)^{n}}{n!}=sum_{n=0}^{infty}frac{(a)^{n}}{n!}b^{n}=1+(a)b+left(frac{a^{2}}{2}right)b^{2}+left(frac{a^{3}}{6}right)b^{3}+cdots$$
At this point I don't know how to proceed. I don't see how I could get b out of the series and have it as a power that the series is raised to. I thought that it resembled the geometric series, but that did not seem to lead anywhere.
sequences-and-series proof-writing exponential-function
$endgroup$
add a comment |
$begingroup$
I want to prove that
$$e^{ab}=left(e^{a}right)^{b}$$where
$$a,b in mathbb{R}$$ using the infinite series for $$e^{x}$$
My attempt:
$$e^{ab}=sum_{n=0}^{infty}frac{(ab)^{n}}{n!}=sum_{n=0}^{infty}frac{(a)^{n}(b)^{n}}{n!}=sum_{n=0}^{infty}frac{(a)^{n}}{n!}b^{n}=1+(a)b+left(frac{a^{2}}{2}right)b^{2}+left(frac{a^{3}}{6}right)b^{3}+cdots$$
At this point I don't know how to proceed. I don't see how I could get b out of the series and have it as a power that the series is raised to. I thought that it resembled the geometric series, but that did not seem to lead anywhere.
sequences-and-series proof-writing exponential-function
$endgroup$
$begingroup$
Please see math.meta.stackexchange.com/questions/5020
$endgroup$
– Lord Shark the Unknown
Dec 30 '18 at 17:23
2
$begingroup$
Unless I'm blind, isn't $n^{mk} = (n^m)^k$ true always, whether or not $n=e$?
$endgroup$
– Zachary Hunter
Dec 30 '18 at 17:26
2
$begingroup$
He’s specifically trying to show it for $e$ using the series expansion.
$endgroup$
– KM101
Dec 30 '18 at 17:31
$begingroup$
You are complicating things for yourself by trying to show this using series. This is a general property. See for example math.stackexchange.com/questions/91869/… or math.stackexchange.com/questions/614162/laws-of-indices or math.stackexchange.com/questions/2086109/…
$endgroup$
– Winther
Dec 30 '18 at 17:47
add a comment |
$begingroup$
I want to prove that
$$e^{ab}=left(e^{a}right)^{b}$$where
$$a,b in mathbb{R}$$ using the infinite series for $$e^{x}$$
My attempt:
$$e^{ab}=sum_{n=0}^{infty}frac{(ab)^{n}}{n!}=sum_{n=0}^{infty}frac{(a)^{n}(b)^{n}}{n!}=sum_{n=0}^{infty}frac{(a)^{n}}{n!}b^{n}=1+(a)b+left(frac{a^{2}}{2}right)b^{2}+left(frac{a^{3}}{6}right)b^{3}+cdots$$
At this point I don't know how to proceed. I don't see how I could get b out of the series and have it as a power that the series is raised to. I thought that it resembled the geometric series, but that did not seem to lead anywhere.
sequences-and-series proof-writing exponential-function
$endgroup$
I want to prove that
$$e^{ab}=left(e^{a}right)^{b}$$where
$$a,b in mathbb{R}$$ using the infinite series for $$e^{x}$$
My attempt:
$$e^{ab}=sum_{n=0}^{infty}frac{(ab)^{n}}{n!}=sum_{n=0}^{infty}frac{(a)^{n}(b)^{n}}{n!}=sum_{n=0}^{infty}frac{(a)^{n}}{n!}b^{n}=1+(a)b+left(frac{a^{2}}{2}right)b^{2}+left(frac{a^{3}}{6}right)b^{3}+cdots$$
At this point I don't know how to proceed. I don't see how I could get b out of the series and have it as a power that the series is raised to. I thought that it resembled the geometric series, but that did not seem to lead anywhere.
sequences-and-series proof-writing exponential-function
sequences-and-series proof-writing exponential-function
edited Dec 30 '18 at 17:41
Blue
47.8k870152
47.8k870152
asked Dec 30 '18 at 17:22
bhjkbhjk
112
112
$begingroup$
Please see math.meta.stackexchange.com/questions/5020
$endgroup$
– Lord Shark the Unknown
Dec 30 '18 at 17:23
2
$begingroup$
Unless I'm blind, isn't $n^{mk} = (n^m)^k$ true always, whether or not $n=e$?
$endgroup$
– Zachary Hunter
Dec 30 '18 at 17:26
2
$begingroup$
He’s specifically trying to show it for $e$ using the series expansion.
$endgroup$
– KM101
Dec 30 '18 at 17:31
$begingroup$
You are complicating things for yourself by trying to show this using series. This is a general property. See for example math.stackexchange.com/questions/91869/… or math.stackexchange.com/questions/614162/laws-of-indices or math.stackexchange.com/questions/2086109/…
$endgroup$
– Winther
Dec 30 '18 at 17:47
add a comment |
$begingroup$
Please see math.meta.stackexchange.com/questions/5020
$endgroup$
– Lord Shark the Unknown
Dec 30 '18 at 17:23
2
$begingroup$
Unless I'm blind, isn't $n^{mk} = (n^m)^k$ true always, whether or not $n=e$?
$endgroup$
– Zachary Hunter
Dec 30 '18 at 17:26
2
$begingroup$
He’s specifically trying to show it for $e$ using the series expansion.
$endgroup$
– KM101
Dec 30 '18 at 17:31
$begingroup$
You are complicating things for yourself by trying to show this using series. This is a general property. See for example math.stackexchange.com/questions/91869/… or math.stackexchange.com/questions/614162/laws-of-indices or math.stackexchange.com/questions/2086109/…
$endgroup$
– Winther
Dec 30 '18 at 17:47
$begingroup$
Please see math.meta.stackexchange.com/questions/5020
$endgroup$
– Lord Shark the Unknown
Dec 30 '18 at 17:23
$begingroup$
Please see math.meta.stackexchange.com/questions/5020
$endgroup$
– Lord Shark the Unknown
Dec 30 '18 at 17:23
2
2
$begingroup$
Unless I'm blind, isn't $n^{mk} = (n^m)^k$ true always, whether or not $n=e$?
$endgroup$
– Zachary Hunter
Dec 30 '18 at 17:26
$begingroup$
Unless I'm blind, isn't $n^{mk} = (n^m)^k$ true always, whether or not $n=e$?
$endgroup$
– Zachary Hunter
Dec 30 '18 at 17:26
2
2
$begingroup$
He’s specifically trying to show it for $e$ using the series expansion.
$endgroup$
– KM101
Dec 30 '18 at 17:31
$begingroup$
He’s specifically trying to show it for $e$ using the series expansion.
$endgroup$
– KM101
Dec 30 '18 at 17:31
$begingroup$
You are complicating things for yourself by trying to show this using series. This is a general property. See for example math.stackexchange.com/questions/91869/… or math.stackexchange.com/questions/614162/laws-of-indices or math.stackexchange.com/questions/2086109/…
$endgroup$
– Winther
Dec 30 '18 at 17:47
$begingroup$
You are complicating things for yourself by trying to show this using series. This is a general property. See for example math.stackexchange.com/questions/91869/… or math.stackexchange.com/questions/614162/laws-of-indices or math.stackexchange.com/questions/2086109/…
$endgroup$
– Winther
Dec 30 '18 at 17:47
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
The problem with trying to prove this is that you need a rigorous definition for $x^y$ where $x$ can be any positive number and $y$ can be any real number. Now, we already have a rigorous definition for $exp(y)=e^y$ for any $yinBbb{R}$ as $sum_{n=0}^infty frac{y^n}{n!}$. Therefore, I usually people write $x^y$ in terms of the exponential function:
$$x^y=exp(yln x)$$
Thus, the easiest way to prove $(e^a)^b$ is to just use this definition of $x^y$:
$$(e^a)^b=exp(bln e^a)=exp(ba)=e^{ab}$$
I know that's not really the series solution you wanted, but I still think it is helpful to see this kind of proof.
$endgroup$
$begingroup$
How would you go about proving then that ln((e^a)^b) = b*ln(e^a)?
$endgroup$
– bhjk
Dec 30 '18 at 17:46
$begingroup$
@bhjk: That follows directly from the definition of $x^y$ presented here.
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:48
2
$begingroup$
Note that this definition doesn't really work "where $x$ and $y$ can be any real number", but only where $x$ is a positive real (and $y$ is an arbitrary real). (Irrational powers of negative reals are usually not considered well-defined at all).
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:51
add a comment |
$begingroup$
If we assume the "meaning" of $e^a$ and if $bin mathbb{Q}$, $b=frac{p}{q}$, then we know what does it mean to $textit{raise a number by the power $p$ and taking the $q-th$ root of that.}$ That is to say, $(e^a)^p=e^a...e^a=e^{ap}$, and $(e^{ap})^{frac{1}{q}}$ is a number which gives $e^{ap}$ when multiplied $q$-times, and we are done.
Now if the number $b$ is not a rational, then consider the following number (can be defined in this way, uses l.u.b. property of $mathbb{R}$) $$(e^a)^b=sup{(e^a)^x,,|,, xin mathbb{Q}text{ and }x<b}=sup{e^{ax},,|,, xin mathbb{Q}text{ and }x<b}=e^{ab}.$$
$endgroup$
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%2f3057028%2fproof-that-eab-leftea-rightb-using-the-series-for-ex%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
The problem with trying to prove this is that you need a rigorous definition for $x^y$ where $x$ can be any positive number and $y$ can be any real number. Now, we already have a rigorous definition for $exp(y)=e^y$ for any $yinBbb{R}$ as $sum_{n=0}^infty frac{y^n}{n!}$. Therefore, I usually people write $x^y$ in terms of the exponential function:
$$x^y=exp(yln x)$$
Thus, the easiest way to prove $(e^a)^b$ is to just use this definition of $x^y$:
$$(e^a)^b=exp(bln e^a)=exp(ba)=e^{ab}$$
I know that's not really the series solution you wanted, but I still think it is helpful to see this kind of proof.
$endgroup$
$begingroup$
How would you go about proving then that ln((e^a)^b) = b*ln(e^a)?
$endgroup$
– bhjk
Dec 30 '18 at 17:46
$begingroup$
@bhjk: That follows directly from the definition of $x^y$ presented here.
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:48
2
$begingroup$
Note that this definition doesn't really work "where $x$ and $y$ can be any real number", but only where $x$ is a positive real (and $y$ is an arbitrary real). (Irrational powers of negative reals are usually not considered well-defined at all).
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:51
add a comment |
$begingroup$
The problem with trying to prove this is that you need a rigorous definition for $x^y$ where $x$ can be any positive number and $y$ can be any real number. Now, we already have a rigorous definition for $exp(y)=e^y$ for any $yinBbb{R}$ as $sum_{n=0}^infty frac{y^n}{n!}$. Therefore, I usually people write $x^y$ in terms of the exponential function:
$$x^y=exp(yln x)$$
Thus, the easiest way to prove $(e^a)^b$ is to just use this definition of $x^y$:
$$(e^a)^b=exp(bln e^a)=exp(ba)=e^{ab}$$
I know that's not really the series solution you wanted, but I still think it is helpful to see this kind of proof.
$endgroup$
$begingroup$
How would you go about proving then that ln((e^a)^b) = b*ln(e^a)?
$endgroup$
– bhjk
Dec 30 '18 at 17:46
$begingroup$
@bhjk: That follows directly from the definition of $x^y$ presented here.
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:48
2
$begingroup$
Note that this definition doesn't really work "where $x$ and $y$ can be any real number", but only where $x$ is a positive real (and $y$ is an arbitrary real). (Irrational powers of negative reals are usually not considered well-defined at all).
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:51
add a comment |
$begingroup$
The problem with trying to prove this is that you need a rigorous definition for $x^y$ where $x$ can be any positive number and $y$ can be any real number. Now, we already have a rigorous definition for $exp(y)=e^y$ for any $yinBbb{R}$ as $sum_{n=0}^infty frac{y^n}{n!}$. Therefore, I usually people write $x^y$ in terms of the exponential function:
$$x^y=exp(yln x)$$
Thus, the easiest way to prove $(e^a)^b$ is to just use this definition of $x^y$:
$$(e^a)^b=exp(bln e^a)=exp(ba)=e^{ab}$$
I know that's not really the series solution you wanted, but I still think it is helpful to see this kind of proof.
$endgroup$
The problem with trying to prove this is that you need a rigorous definition for $x^y$ where $x$ can be any positive number and $y$ can be any real number. Now, we already have a rigorous definition for $exp(y)=e^y$ for any $yinBbb{R}$ as $sum_{n=0}^infty frac{y^n}{n!}$. Therefore, I usually people write $x^y$ in terms of the exponential function:
$$x^y=exp(yln x)$$
Thus, the easiest way to prove $(e^a)^b$ is to just use this definition of $x^y$:
$$(e^a)^b=exp(bln e^a)=exp(ba)=e^{ab}$$
I know that's not really the series solution you wanted, but I still think it is helpful to see this kind of proof.
edited Dec 30 '18 at 18:25
answered Dec 30 '18 at 17:39
Noble MushtakNoble Mushtak
15.2k1735
15.2k1735
$begingroup$
How would you go about proving then that ln((e^a)^b) = b*ln(e^a)?
$endgroup$
– bhjk
Dec 30 '18 at 17:46
$begingroup$
@bhjk: That follows directly from the definition of $x^y$ presented here.
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:48
2
$begingroup$
Note that this definition doesn't really work "where $x$ and $y$ can be any real number", but only where $x$ is a positive real (and $y$ is an arbitrary real). (Irrational powers of negative reals are usually not considered well-defined at all).
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:51
add a comment |
$begingroup$
How would you go about proving then that ln((e^a)^b) = b*ln(e^a)?
$endgroup$
– bhjk
Dec 30 '18 at 17:46
$begingroup$
@bhjk: That follows directly from the definition of $x^y$ presented here.
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:48
2
$begingroup$
Note that this definition doesn't really work "where $x$ and $y$ can be any real number", but only where $x$ is a positive real (and $y$ is an arbitrary real). (Irrational powers of negative reals are usually not considered well-defined at all).
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:51
$begingroup$
How would you go about proving then that ln((e^a)^b) = b*ln(e^a)?
$endgroup$
– bhjk
Dec 30 '18 at 17:46
$begingroup$
How would you go about proving then that ln((e^a)^b) = b*ln(e^a)?
$endgroup$
– bhjk
Dec 30 '18 at 17:46
$begingroup$
@bhjk: That follows directly from the definition of $x^y$ presented here.
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:48
$begingroup$
@bhjk: That follows directly from the definition of $x^y$ presented here.
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:48
2
2
$begingroup$
Note that this definition doesn't really work "where $x$ and $y$ can be any real number", but only where $x$ is a positive real (and $y$ is an arbitrary real). (Irrational powers of negative reals are usually not considered well-defined at all).
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:51
$begingroup$
Note that this definition doesn't really work "where $x$ and $y$ can be any real number", but only where $x$ is a positive real (and $y$ is an arbitrary real). (Irrational powers of negative reals are usually not considered well-defined at all).
$endgroup$
– Henning Makholm
Dec 30 '18 at 17:51
add a comment |
$begingroup$
If we assume the "meaning" of $e^a$ and if $bin mathbb{Q}$, $b=frac{p}{q}$, then we know what does it mean to $textit{raise a number by the power $p$ and taking the $q-th$ root of that.}$ That is to say, $(e^a)^p=e^a...e^a=e^{ap}$, and $(e^{ap})^{frac{1}{q}}$ is a number which gives $e^{ap}$ when multiplied $q$-times, and we are done.
Now if the number $b$ is not a rational, then consider the following number (can be defined in this way, uses l.u.b. property of $mathbb{R}$) $$(e^a)^b=sup{(e^a)^x,,|,, xin mathbb{Q}text{ and }x<b}=sup{e^{ax},,|,, xin mathbb{Q}text{ and }x<b}=e^{ab}.$$
$endgroup$
add a comment |
$begingroup$
If we assume the "meaning" of $e^a$ and if $bin mathbb{Q}$, $b=frac{p}{q}$, then we know what does it mean to $textit{raise a number by the power $p$ and taking the $q-th$ root of that.}$ That is to say, $(e^a)^p=e^a...e^a=e^{ap}$, and $(e^{ap})^{frac{1}{q}}$ is a number which gives $e^{ap}$ when multiplied $q$-times, and we are done.
Now if the number $b$ is not a rational, then consider the following number (can be defined in this way, uses l.u.b. property of $mathbb{R}$) $$(e^a)^b=sup{(e^a)^x,,|,, xin mathbb{Q}text{ and }x<b}=sup{e^{ax},,|,, xin mathbb{Q}text{ and }x<b}=e^{ab}.$$
$endgroup$
add a comment |
$begingroup$
If we assume the "meaning" of $e^a$ and if $bin mathbb{Q}$, $b=frac{p}{q}$, then we know what does it mean to $textit{raise a number by the power $p$ and taking the $q-th$ root of that.}$ That is to say, $(e^a)^p=e^a...e^a=e^{ap}$, and $(e^{ap})^{frac{1}{q}}$ is a number which gives $e^{ap}$ when multiplied $q$-times, and we are done.
Now if the number $b$ is not a rational, then consider the following number (can be defined in this way, uses l.u.b. property of $mathbb{R}$) $$(e^a)^b=sup{(e^a)^x,,|,, xin mathbb{Q}text{ and }x<b}=sup{e^{ax},,|,, xin mathbb{Q}text{ and }x<b}=e^{ab}.$$
$endgroup$
If we assume the "meaning" of $e^a$ and if $bin mathbb{Q}$, $b=frac{p}{q}$, then we know what does it mean to $textit{raise a number by the power $p$ and taking the $q-th$ root of that.}$ That is to say, $(e^a)^p=e^a...e^a=e^{ap}$, and $(e^{ap})^{frac{1}{q}}$ is a number which gives $e^{ap}$ when multiplied $q$-times, and we are done.
Now if the number $b$ is not a rational, then consider the following number (can be defined in this way, uses l.u.b. property of $mathbb{R}$) $$(e^a)^b=sup{(e^a)^x,,|,, xin mathbb{Q}text{ and }x<b}=sup{e^{ax},,|,, xin mathbb{Q}text{ and }x<b}=e^{ab}.$$
edited Dec 30 '18 at 18:21
answered Dec 30 '18 at 18:14
DèöDèö
16113
16113
add a comment |
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%2f3057028%2fproof-that-eab-leftea-rightb-using-the-series-for-ex%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
$begingroup$
Please see math.meta.stackexchange.com/questions/5020
$endgroup$
– Lord Shark the Unknown
Dec 30 '18 at 17:23
2
$begingroup$
Unless I'm blind, isn't $n^{mk} = (n^m)^k$ true always, whether or not $n=e$?
$endgroup$
– Zachary Hunter
Dec 30 '18 at 17:26
2
$begingroup$
He’s specifically trying to show it for $e$ using the series expansion.
$endgroup$
– KM101
Dec 30 '18 at 17:31
$begingroup$
You are complicating things for yourself by trying to show this using series. This is a general property. See for example math.stackexchange.com/questions/91869/… or math.stackexchange.com/questions/614162/laws-of-indices or math.stackexchange.com/questions/2086109/…
$endgroup$
– Winther
Dec 30 '18 at 17:47