Using mean value theorem to show that $cos (x)>1-x^2/2$
$begingroup$
I have a question, by applying the mean value theorem to $f(x)=frac{x^2}{2}+cos (x)$, on the interval $[0,x]$, show that $cos (x)>1-frac{x^2}{2}$.
We know that $frac{text{df}(x)}{text{dx}}=x-sin (x)$, for $x>0$. By the MVT, if $x>0$, then $f(x)-f(0)=(x+0) f'(c)$ for some $c>0$.
This is where I get confused:
so, $f(x)>f(0)=1$, but why? Is it my lack of inequality that is showing, or what am I missing? Is $f'(x)cdot x=1$ or what is going on?
real-analysis inequality
$endgroup$
|
show 4 more comments
$begingroup$
I have a question, by applying the mean value theorem to $f(x)=frac{x^2}{2}+cos (x)$, on the interval $[0,x]$, show that $cos (x)>1-frac{x^2}{2}$.
We know that $frac{text{df}(x)}{text{dx}}=x-sin (x)$, for $x>0$. By the MVT, if $x>0$, then $f(x)-f(0)=(x+0) f'(c)$ for some $c>0$.
This is where I get confused:
so, $f(x)>f(0)=1$, but why? Is it my lack of inequality that is showing, or what am I missing? Is $f'(x)cdot x=1$ or what is going on?
real-analysis inequality
$endgroup$
$begingroup$
The sign of $f'(c)$ gives the result (and the approach works for $xlt0$ as well).
$endgroup$
– Did
Aug 5 '14 at 9:47
$begingroup$
Meaning that f′(c) is positive?
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:50
$begingroup$
Tell me. $ $ $ $
$endgroup$
– Did
Aug 5 '14 at 9:53
$begingroup$
@Did Im not sure if I got it? Why exactly equal to 1? and is it the positive f′(c)* positive x = positive __
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:57
1
$begingroup$
see it as $f(x)>[f(0)=1]$
$endgroup$
– RE60K
Aug 5 '14 at 11:17
|
show 4 more comments
$begingroup$
I have a question, by applying the mean value theorem to $f(x)=frac{x^2}{2}+cos (x)$, on the interval $[0,x]$, show that $cos (x)>1-frac{x^2}{2}$.
We know that $frac{text{df}(x)}{text{dx}}=x-sin (x)$, for $x>0$. By the MVT, if $x>0$, then $f(x)-f(0)=(x+0) f'(c)$ for some $c>0$.
This is where I get confused:
so, $f(x)>f(0)=1$, but why? Is it my lack of inequality that is showing, or what am I missing? Is $f'(x)cdot x=1$ or what is going on?
real-analysis inequality
$endgroup$
I have a question, by applying the mean value theorem to $f(x)=frac{x^2}{2}+cos (x)$, on the interval $[0,x]$, show that $cos (x)>1-frac{x^2}{2}$.
We know that $frac{text{df}(x)}{text{dx}}=x-sin (x)$, for $x>0$. By the MVT, if $x>0$, then $f(x)-f(0)=(x+0) f'(c)$ for some $c>0$.
This is where I get confused:
so, $f(x)>f(0)=1$, but why? Is it my lack of inequality that is showing, or what am I missing? Is $f'(x)cdot x=1$ or what is going on?
real-analysis inequality
real-analysis inequality
edited Jan 2 at 17:57
amWhy
1
1
asked Aug 5 '14 at 9:45
ALEXANDERALEXANDER
8951921
8951921
$begingroup$
The sign of $f'(c)$ gives the result (and the approach works for $xlt0$ as well).
$endgroup$
– Did
Aug 5 '14 at 9:47
$begingroup$
Meaning that f′(c) is positive?
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:50
$begingroup$
Tell me. $ $ $ $
$endgroup$
– Did
Aug 5 '14 at 9:53
$begingroup$
@Did Im not sure if I got it? Why exactly equal to 1? and is it the positive f′(c)* positive x = positive __
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:57
1
$begingroup$
see it as $f(x)>[f(0)=1]$
$endgroup$
– RE60K
Aug 5 '14 at 11:17
|
show 4 more comments
$begingroup$
The sign of $f'(c)$ gives the result (and the approach works for $xlt0$ as well).
$endgroup$
– Did
Aug 5 '14 at 9:47
$begingroup$
Meaning that f′(c) is positive?
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:50
$begingroup$
Tell me. $ $ $ $
$endgroup$
– Did
Aug 5 '14 at 9:53
$begingroup$
@Did Im not sure if I got it? Why exactly equal to 1? and is it the positive f′(c)* positive x = positive __
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:57
1
$begingroup$
see it as $f(x)>[f(0)=1]$
$endgroup$
– RE60K
Aug 5 '14 at 11:17
$begingroup$
The sign of $f'(c)$ gives the result (and the approach works for $xlt0$ as well).
$endgroup$
– Did
Aug 5 '14 at 9:47
$begingroup$
The sign of $f'(c)$ gives the result (and the approach works for $xlt0$ as well).
$endgroup$
– Did
Aug 5 '14 at 9:47
$begingroup$
Meaning that f′(c) is positive?
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:50
$begingroup$
Meaning that f′(c) is positive?
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:50
$begingroup$
Tell me. $ $ $ $
$endgroup$
– Did
Aug 5 '14 at 9:53
$begingroup$
Tell me. $ $ $ $
$endgroup$
– Did
Aug 5 '14 at 9:53
$begingroup$
@Did Im not sure if I got it? Why exactly equal to 1? and is it the positive f′(c)* positive x = positive __
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:57
$begingroup$
@Did Im not sure if I got it? Why exactly equal to 1? and is it the positive f′(c)* positive x = positive __
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:57
1
1
$begingroup$
see it as $f(x)>[f(0)=1]$
$endgroup$
– RE60K
Aug 5 '14 at 11:17
$begingroup$
see it as $f(x)>[f(0)=1]$
$endgroup$
– RE60K
Aug 5 '14 at 11:17
|
show 4 more comments
2 Answers
2
active
oldest
votes
$begingroup$
$ f(x)=x^2/2+cos(x)$
Note that $f(0)=0^2/2+1=1$
From your equation:
$$f(x)-f(0)=(x)f'(c)=x(c-sin c)$$
Let $g(x)=x-sin x$
Again you can show that $g'(x)=1-cos x$ which is always greater than
$0$ due to bounded nature of $cos x$.As $g(0)=0$ and it is an
increasing function ${g'(x)>0;forall x>0$}, thus $g(x)>0 ;forall x>0$.
So $f(x)-f(0)>0;forall x>0$ as $x>0$ and $c-sin c >0;forall c>0${as $0<c<x$}.
So $f(x)>f(0)=1$
$endgroup$
add a comment |
$begingroup$
You started off well.
Notice that, by MVT:
$$f'(c) = frac{f(x) - f(0)}{x - 0}$$
S0
$$xf'(c) = f(x) - f(0)$$
Notice that x is positive, and since $$f'(x) = x - sin(x)$$
Also, note that $x > sin(x)$, so $f'(x) > 0$
Therefore,
We can conclude that
$$f(x) > f(0)$$
And
$$cos(x) > 1- frac{x^2}{2}$$
$endgroup$
$begingroup$
Why not factor out the constant in the derivative
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:16
$begingroup$
@ALEXANDER which constant are you referring to, $c$? That's simply a value of x that makes this equality true for the MVT. You cannot take it out
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:17
$begingroup$
@ALEXANDER all you show is that $f'(x) > 0$, so you can that $f(x) - f(0) > 0$, so that $f(x) > f(0)$
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:19
$begingroup$
You are getting the derivative of x^2/2 to become 2x, but should it not be just x, when you are factoring out the constant
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:19
$begingroup$
That I can see, but I do not get where the number 1 is coming from.
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:22
|
show 7 more comments
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
$ f(x)=x^2/2+cos(x)$
Note that $f(0)=0^2/2+1=1$
From your equation:
$$f(x)-f(0)=(x)f'(c)=x(c-sin c)$$
Let $g(x)=x-sin x$
Again you can show that $g'(x)=1-cos x$ which is always greater than
$0$ due to bounded nature of $cos x$.As $g(0)=0$ and it is an
increasing function ${g'(x)>0;forall x>0$}, thus $g(x)>0 ;forall x>0$.
So $f(x)-f(0)>0;forall x>0$ as $x>0$ and $c-sin c >0;forall c>0${as $0<c<x$}.
So $f(x)>f(0)=1$
$endgroup$
add a comment |
$begingroup$
$ f(x)=x^2/2+cos(x)$
Note that $f(0)=0^2/2+1=1$
From your equation:
$$f(x)-f(0)=(x)f'(c)=x(c-sin c)$$
Let $g(x)=x-sin x$
Again you can show that $g'(x)=1-cos x$ which is always greater than
$0$ due to bounded nature of $cos x$.As $g(0)=0$ and it is an
increasing function ${g'(x)>0;forall x>0$}, thus $g(x)>0 ;forall x>0$.
So $f(x)-f(0)>0;forall x>0$ as $x>0$ and $c-sin c >0;forall c>0${as $0<c<x$}.
So $f(x)>f(0)=1$
$endgroup$
add a comment |
$begingroup$
$ f(x)=x^2/2+cos(x)$
Note that $f(0)=0^2/2+1=1$
From your equation:
$$f(x)-f(0)=(x)f'(c)=x(c-sin c)$$
Let $g(x)=x-sin x$
Again you can show that $g'(x)=1-cos x$ which is always greater than
$0$ due to bounded nature of $cos x$.As $g(0)=0$ and it is an
increasing function ${g'(x)>0;forall x>0$}, thus $g(x)>0 ;forall x>0$.
So $f(x)-f(0)>0;forall x>0$ as $x>0$ and $c-sin c >0;forall c>0${as $0<c<x$}.
So $f(x)>f(0)=1$
$endgroup$
$ f(x)=x^2/2+cos(x)$
Note that $f(0)=0^2/2+1=1$
From your equation:
$$f(x)-f(0)=(x)f'(c)=x(c-sin c)$$
Let $g(x)=x-sin x$
Again you can show that $g'(x)=1-cos x$ which is always greater than
$0$ due to bounded nature of $cos x$.As $g(0)=0$ and it is an
increasing function ${g'(x)>0;forall x>0$}, thus $g(x)>0 ;forall x>0$.
So $f(x)-f(0)>0;forall x>0$ as $x>0$ and $c-sin c >0;forall c>0${as $0<c<x$}.
So $f(x)>f(0)=1$
edited Aug 5 '14 at 11:09
Martin Sleziak
44.8k10119272
44.8k10119272
answered Aug 5 '14 at 10:58
RE60KRE60K
14k22155
14k22155
add a comment |
add a comment |
$begingroup$
You started off well.
Notice that, by MVT:
$$f'(c) = frac{f(x) - f(0)}{x - 0}$$
S0
$$xf'(c) = f(x) - f(0)$$
Notice that x is positive, and since $$f'(x) = x - sin(x)$$
Also, note that $x > sin(x)$, so $f'(x) > 0$
Therefore,
We can conclude that
$$f(x) > f(0)$$
And
$$cos(x) > 1- frac{x^2}{2}$$
$endgroup$
$begingroup$
Why not factor out the constant in the derivative
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:16
$begingroup$
@ALEXANDER which constant are you referring to, $c$? That's simply a value of x that makes this equality true for the MVT. You cannot take it out
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:17
$begingroup$
@ALEXANDER all you show is that $f'(x) > 0$, so you can that $f(x) - f(0) > 0$, so that $f(x) > f(0)$
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:19
$begingroup$
You are getting the derivative of x^2/2 to become 2x, but should it not be just x, when you are factoring out the constant
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:19
$begingroup$
That I can see, but I do not get where the number 1 is coming from.
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:22
|
show 7 more comments
$begingroup$
You started off well.
Notice that, by MVT:
$$f'(c) = frac{f(x) - f(0)}{x - 0}$$
S0
$$xf'(c) = f(x) - f(0)$$
Notice that x is positive, and since $$f'(x) = x - sin(x)$$
Also, note that $x > sin(x)$, so $f'(x) > 0$
Therefore,
We can conclude that
$$f(x) > f(0)$$
And
$$cos(x) > 1- frac{x^2}{2}$$
$endgroup$
$begingroup$
Why not factor out the constant in the derivative
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:16
$begingroup$
@ALEXANDER which constant are you referring to, $c$? That's simply a value of x that makes this equality true for the MVT. You cannot take it out
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:17
$begingroup$
@ALEXANDER all you show is that $f'(x) > 0$, so you can that $f(x) - f(0) > 0$, so that $f(x) > f(0)$
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:19
$begingroup$
You are getting the derivative of x^2/2 to become 2x, but should it not be just x, when you are factoring out the constant
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:19
$begingroup$
That I can see, but I do not get where the number 1 is coming from.
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:22
|
show 7 more comments
$begingroup$
You started off well.
Notice that, by MVT:
$$f'(c) = frac{f(x) - f(0)}{x - 0}$$
S0
$$xf'(c) = f(x) - f(0)$$
Notice that x is positive, and since $$f'(x) = x - sin(x)$$
Also, note that $x > sin(x)$, so $f'(x) > 0$
Therefore,
We can conclude that
$$f(x) > f(0)$$
And
$$cos(x) > 1- frac{x^2}{2}$$
$endgroup$
You started off well.
Notice that, by MVT:
$$f'(c) = frac{f(x) - f(0)}{x - 0}$$
S0
$$xf'(c) = f(x) - f(0)$$
Notice that x is positive, and since $$f'(x) = x - sin(x)$$
Also, note that $x > sin(x)$, so $f'(x) > 0$
Therefore,
We can conclude that
$$f(x) > f(0)$$
And
$$cos(x) > 1- frac{x^2}{2}$$
edited Aug 5 '14 at 10:32
answered Aug 5 '14 at 10:15
Varun IyerVarun Iyer
5,317826
5,317826
$begingroup$
Why not factor out the constant in the derivative
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:16
$begingroup$
@ALEXANDER which constant are you referring to, $c$? That's simply a value of x that makes this equality true for the MVT. You cannot take it out
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:17
$begingroup$
@ALEXANDER all you show is that $f'(x) > 0$, so you can that $f(x) - f(0) > 0$, so that $f(x) > f(0)$
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:19
$begingroup$
You are getting the derivative of x^2/2 to become 2x, but should it not be just x, when you are factoring out the constant
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:19
$begingroup$
That I can see, but I do not get where the number 1 is coming from.
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:22
|
show 7 more comments
$begingroup$
Why not factor out the constant in the derivative
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:16
$begingroup$
@ALEXANDER which constant are you referring to, $c$? That's simply a value of x that makes this equality true for the MVT. You cannot take it out
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:17
$begingroup$
@ALEXANDER all you show is that $f'(x) > 0$, so you can that $f(x) - f(0) > 0$, so that $f(x) > f(0)$
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:19
$begingroup$
You are getting the derivative of x^2/2 to become 2x, but should it not be just x, when you are factoring out the constant
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:19
$begingroup$
That I can see, but I do not get where the number 1 is coming from.
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:22
$begingroup$
Why not factor out the constant in the derivative
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:16
$begingroup$
Why not factor out the constant in the derivative
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:16
$begingroup$
@ALEXANDER which constant are you referring to, $c$? That's simply a value of x that makes this equality true for the MVT. You cannot take it out
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:17
$begingroup$
@ALEXANDER which constant are you referring to, $c$? That's simply a value of x that makes this equality true for the MVT. You cannot take it out
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:17
$begingroup$
@ALEXANDER all you show is that $f'(x) > 0$, so you can that $f(x) - f(0) > 0$, so that $f(x) > f(0)$
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:19
$begingroup$
@ALEXANDER all you show is that $f'(x) > 0$, so you can that $f(x) - f(0) > 0$, so that $f(x) > f(0)$
$endgroup$
– Varun Iyer
Aug 5 '14 at 10:19
$begingroup$
You are getting the derivative of x^2/2 to become 2x, but should it not be just x, when you are factoring out the constant
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:19
$begingroup$
You are getting the derivative of x^2/2 to become 2x, but should it not be just x, when you are factoring out the constant
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:19
$begingroup$
That I can see, but I do not get where the number 1 is coming from.
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:22
$begingroup$
That I can see, but I do not get where the number 1 is coming from.
$endgroup$
– ALEXANDER
Aug 5 '14 at 10:22
|
show 7 more comments
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$begingroup$
The sign of $f'(c)$ gives the result (and the approach works for $xlt0$ as well).
$endgroup$
– Did
Aug 5 '14 at 9:47
$begingroup$
Meaning that f′(c) is positive?
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:50
$begingroup$
Tell me. $ $ $ $
$endgroup$
– Did
Aug 5 '14 at 9:53
$begingroup$
@Did Im not sure if I got it? Why exactly equal to 1? and is it the positive f′(c)* positive x = positive __
$endgroup$
– ALEXANDER
Aug 5 '14 at 9:57
1
$begingroup$
see it as $f(x)>[f(0)=1]$
$endgroup$
– RE60K
Aug 5 '14 at 11:17