Proof that $tan(x)leq{}x-frac{pi}{4}+tan(frac{pi}{4})$ using the Mean Value Theorem
$begingroup$
We're asked to proof the above inequality $forall{}xin(frac{-pi}{2},frac{pi}{4}]$. Although am a bit confused with the fact that $tan(frac{-pi}{2})$ is undefined. And thus am stuck when applying the mean value theorem to proof the statement. What interval should I be taking in order to apply the theorem? Thanks in advance.
My attempt:
Let $f(t)=tan(t)$ such that $tin(frac{-pi}{2},x]$ whereas $xleq{}frac{pi}{4}$ then, because $f$ is continuous on the closed interval and differentiable on the open interval, $exists{}cin(-frac{pi}{2},x)$ such that the difference of the quotient is equal to $f'(c)$ but then again, I can't take $f(-frac{pi}{2})$
real-analysis calculus analysis proof-writing
$endgroup$
add a comment |
$begingroup$
We're asked to proof the above inequality $forall{}xin(frac{-pi}{2},frac{pi}{4}]$. Although am a bit confused with the fact that $tan(frac{-pi}{2})$ is undefined. And thus am stuck when applying the mean value theorem to proof the statement. What interval should I be taking in order to apply the theorem? Thanks in advance.
My attempt:
Let $f(t)=tan(t)$ such that $tin(frac{-pi}{2},x]$ whereas $xleq{}frac{pi}{4}$ then, because $f$ is continuous on the closed interval and differentiable on the open interval, $exists{}cin(-frac{pi}{2},x)$ such that the difference of the quotient is equal to $f'(c)$ but then again, I can't take $f(-frac{pi}{2})$
real-analysis calculus analysis proof-writing
$endgroup$
add a comment |
$begingroup$
We're asked to proof the above inequality $forall{}xin(frac{-pi}{2},frac{pi}{4}]$. Although am a bit confused with the fact that $tan(frac{-pi}{2})$ is undefined. And thus am stuck when applying the mean value theorem to proof the statement. What interval should I be taking in order to apply the theorem? Thanks in advance.
My attempt:
Let $f(t)=tan(t)$ such that $tin(frac{-pi}{2},x]$ whereas $xleq{}frac{pi}{4}$ then, because $f$ is continuous on the closed interval and differentiable on the open interval, $exists{}cin(-frac{pi}{2},x)$ such that the difference of the quotient is equal to $f'(c)$ but then again, I can't take $f(-frac{pi}{2})$
real-analysis calculus analysis proof-writing
$endgroup$
We're asked to proof the above inequality $forall{}xin(frac{-pi}{2},frac{pi}{4}]$. Although am a bit confused with the fact that $tan(frac{-pi}{2})$ is undefined. And thus am stuck when applying the mean value theorem to proof the statement. What interval should I be taking in order to apply the theorem? Thanks in advance.
My attempt:
Let $f(t)=tan(t)$ such that $tin(frac{-pi}{2},x]$ whereas $xleq{}frac{pi}{4}$ then, because $f$ is continuous on the closed interval and differentiable on the open interval, $exists{}cin(-frac{pi}{2},x)$ such that the difference of the quotient is equal to $f'(c)$ but then again, I can't take $f(-frac{pi}{2})$
real-analysis calculus analysis proof-writing
real-analysis calculus analysis proof-writing
edited Dec 9 '18 at 20:16
kareem bokai
asked Dec 9 '18 at 20:01
kareem bokaikareem bokai
416
416
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
If $x=fracpi4$, then $tan(x)=x-fracpi4+tanleft(fracpi4right)$. On the other and, if $xinleft(-fracpi2,fracpi4right)$, then$$frac{tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)}{x-fracpi4}=tan'(c)-1,$$for some $c$ between $x$ and $fracpi4$. But $tan'(c)-1+=tan^2(c)>0$. Since $x-fracpi4<0$, you deduce that $tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)<0$. In other words, $ tan(x)<x-fracpi4+tanleft(fracpi4right)$.
$endgroup$
$begingroup$
Why are we allowed to take $(x,frac{pi}{4})$ as an interval in this case? I see now that it leads to the right conclusion but am still confused on why we are capable of doing that. But thank you for your reply :)
$endgroup$
– kareem bokai
Dec 9 '18 at 20:27
add a comment |
$begingroup$
This is just a variant on José Carlos Santos's answer, using the Mean Value Theorem on the function $tan x$ instead of $tan x-x$.
Rewrite the inequality to be proved first as
$${piover4}-xletanleft(piover4right)-tan x$$
and then, since we're only concerned with $xlt{piover4}$ (after noting that the inequality is a trivial equality for $x={piover4}$), as
$$1le{tanleft(piover4right)-tan xover{piover4}-x}$$
Now the Mean Value Theorem tells us that $(f(b)-f(a))/(b-a)=f'(c)$ for some $cin(a,b)$, provided the function $f$ is continuous on $[a,b]$ and differentiable on $(a,b)$, which the tangent function is as long as $-pi/2lt x$ (and, here, less than $pi/4$), so we have
$${tanleft(piover4right)-tan xover{piover4}-x}=sec^2c$$
for some $cin(x,pi/4)$. Since $sec^2cge1$ for all $c$ (in the appropriate domain), we're done.
$endgroup$
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%2f3032903%2fproof-that-tanx-leqx-frac-pi4-tan-frac-pi4-using-the-mean-val%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
$begingroup$
If $x=fracpi4$, then $tan(x)=x-fracpi4+tanleft(fracpi4right)$. On the other and, if $xinleft(-fracpi2,fracpi4right)$, then$$frac{tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)}{x-fracpi4}=tan'(c)-1,$$for some $c$ between $x$ and $fracpi4$. But $tan'(c)-1+=tan^2(c)>0$. Since $x-fracpi4<0$, you deduce that $tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)<0$. In other words, $ tan(x)<x-fracpi4+tanleft(fracpi4right)$.
$endgroup$
$begingroup$
Why are we allowed to take $(x,frac{pi}{4})$ as an interval in this case? I see now that it leads to the right conclusion but am still confused on why we are capable of doing that. But thank you for your reply :)
$endgroup$
– kareem bokai
Dec 9 '18 at 20:27
add a comment |
$begingroup$
If $x=fracpi4$, then $tan(x)=x-fracpi4+tanleft(fracpi4right)$. On the other and, if $xinleft(-fracpi2,fracpi4right)$, then$$frac{tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)}{x-fracpi4}=tan'(c)-1,$$for some $c$ between $x$ and $fracpi4$. But $tan'(c)-1+=tan^2(c)>0$. Since $x-fracpi4<0$, you deduce that $tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)<0$. In other words, $ tan(x)<x-fracpi4+tanleft(fracpi4right)$.
$endgroup$
$begingroup$
Why are we allowed to take $(x,frac{pi}{4})$ as an interval in this case? I see now that it leads to the right conclusion but am still confused on why we are capable of doing that. But thank you for your reply :)
$endgroup$
– kareem bokai
Dec 9 '18 at 20:27
add a comment |
$begingroup$
If $x=fracpi4$, then $tan(x)=x-fracpi4+tanleft(fracpi4right)$. On the other and, if $xinleft(-fracpi2,fracpi4right)$, then$$frac{tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)}{x-fracpi4}=tan'(c)-1,$$for some $c$ between $x$ and $fracpi4$. But $tan'(c)-1+=tan^2(c)>0$. Since $x-fracpi4<0$, you deduce that $tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)<0$. In other words, $ tan(x)<x-fracpi4+tanleft(fracpi4right)$.
$endgroup$
If $x=fracpi4$, then $tan(x)=x-fracpi4+tanleft(fracpi4right)$. On the other and, if $xinleft(-fracpi2,fracpi4right)$, then$$frac{tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)}{x-fracpi4}=tan'(c)-1,$$for some $c$ between $x$ and $fracpi4$. But $tan'(c)-1+=tan^2(c)>0$. Since $x-fracpi4<0$, you deduce that $tan(x)-x-left(tanleft(fracpi4right)-fracpi4right)<0$. In other words, $ tan(x)<x-fracpi4+tanleft(fracpi4right)$.
answered Dec 9 '18 at 20:20
José Carlos SantosJosé Carlos Santos
158k22126228
158k22126228
$begingroup$
Why are we allowed to take $(x,frac{pi}{4})$ as an interval in this case? I see now that it leads to the right conclusion but am still confused on why we are capable of doing that. But thank you for your reply :)
$endgroup$
– kareem bokai
Dec 9 '18 at 20:27
add a comment |
$begingroup$
Why are we allowed to take $(x,frac{pi}{4})$ as an interval in this case? I see now that it leads to the right conclusion but am still confused on why we are capable of doing that. But thank you for your reply :)
$endgroup$
– kareem bokai
Dec 9 '18 at 20:27
$begingroup$
Why are we allowed to take $(x,frac{pi}{4})$ as an interval in this case? I see now that it leads to the right conclusion but am still confused on why we are capable of doing that. But thank you for your reply :)
$endgroup$
– kareem bokai
Dec 9 '18 at 20:27
$begingroup$
Why are we allowed to take $(x,frac{pi}{4})$ as an interval in this case? I see now that it leads to the right conclusion but am still confused on why we are capable of doing that. But thank you for your reply :)
$endgroup$
– kareem bokai
Dec 9 '18 at 20:27
add a comment |
$begingroup$
This is just a variant on José Carlos Santos's answer, using the Mean Value Theorem on the function $tan x$ instead of $tan x-x$.
Rewrite the inequality to be proved first as
$${piover4}-xletanleft(piover4right)-tan x$$
and then, since we're only concerned with $xlt{piover4}$ (after noting that the inequality is a trivial equality for $x={piover4}$), as
$$1le{tanleft(piover4right)-tan xover{piover4}-x}$$
Now the Mean Value Theorem tells us that $(f(b)-f(a))/(b-a)=f'(c)$ for some $cin(a,b)$, provided the function $f$ is continuous on $[a,b]$ and differentiable on $(a,b)$, which the tangent function is as long as $-pi/2lt x$ (and, here, less than $pi/4$), so we have
$${tanleft(piover4right)-tan xover{piover4}-x}=sec^2c$$
for some $cin(x,pi/4)$. Since $sec^2cge1$ for all $c$ (in the appropriate domain), we're done.
$endgroup$
add a comment |
$begingroup$
This is just a variant on José Carlos Santos's answer, using the Mean Value Theorem on the function $tan x$ instead of $tan x-x$.
Rewrite the inequality to be proved first as
$${piover4}-xletanleft(piover4right)-tan x$$
and then, since we're only concerned with $xlt{piover4}$ (after noting that the inequality is a trivial equality for $x={piover4}$), as
$$1le{tanleft(piover4right)-tan xover{piover4}-x}$$
Now the Mean Value Theorem tells us that $(f(b)-f(a))/(b-a)=f'(c)$ for some $cin(a,b)$, provided the function $f$ is continuous on $[a,b]$ and differentiable on $(a,b)$, which the tangent function is as long as $-pi/2lt x$ (and, here, less than $pi/4$), so we have
$${tanleft(piover4right)-tan xover{piover4}-x}=sec^2c$$
for some $cin(x,pi/4)$. Since $sec^2cge1$ for all $c$ (in the appropriate domain), we're done.
$endgroup$
add a comment |
$begingroup$
This is just a variant on José Carlos Santos's answer, using the Mean Value Theorem on the function $tan x$ instead of $tan x-x$.
Rewrite the inequality to be proved first as
$${piover4}-xletanleft(piover4right)-tan x$$
and then, since we're only concerned with $xlt{piover4}$ (after noting that the inequality is a trivial equality for $x={piover4}$), as
$$1le{tanleft(piover4right)-tan xover{piover4}-x}$$
Now the Mean Value Theorem tells us that $(f(b)-f(a))/(b-a)=f'(c)$ for some $cin(a,b)$, provided the function $f$ is continuous on $[a,b]$ and differentiable on $(a,b)$, which the tangent function is as long as $-pi/2lt x$ (and, here, less than $pi/4$), so we have
$${tanleft(piover4right)-tan xover{piover4}-x}=sec^2c$$
for some $cin(x,pi/4)$. Since $sec^2cge1$ for all $c$ (in the appropriate domain), we're done.
$endgroup$
This is just a variant on José Carlos Santos's answer, using the Mean Value Theorem on the function $tan x$ instead of $tan x-x$.
Rewrite the inequality to be proved first as
$${piover4}-xletanleft(piover4right)-tan x$$
and then, since we're only concerned with $xlt{piover4}$ (after noting that the inequality is a trivial equality for $x={piover4}$), as
$$1le{tanleft(piover4right)-tan xover{piover4}-x}$$
Now the Mean Value Theorem tells us that $(f(b)-f(a))/(b-a)=f'(c)$ for some $cin(a,b)$, provided the function $f$ is continuous on $[a,b]$ and differentiable on $(a,b)$, which the tangent function is as long as $-pi/2lt x$ (and, here, less than $pi/4$), so we have
$${tanleft(piover4right)-tan xover{piover4}-x}=sec^2c$$
for some $cin(x,pi/4)$. Since $sec^2cge1$ for all $c$ (in the appropriate domain), we're done.
answered Dec 9 '18 at 20:47
Barry CipraBarry Cipra
59.4k653126
59.4k653126
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%2f3032903%2fproof-that-tanx-leqx-frac-pi4-tan-frac-pi4-using-the-mean-val%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