Given $lim_{x rightarrow a}f(x)+frac1{left|f(x)right|}=0$, what is $lim_{x rightarrow a}f(x)$?

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$begingroup$
Another question I am struggling with:
Let $f:mathbb{R} rightarrow mathbb{R}$ be defined in a neighborhood of $a$ (except maybe $a$ itself) and suppose that, $lim_{x rightarrow a}left(f(x)+dfrac{1}{left|f(x)right|}right)=0$. Find $lim_{x rightarrow a}f(x)$, and prove by definition, that this is indeed the limit.
Thank you all for your answers!!
But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there?
calculus limits
$endgroup$
add a comment |
$begingroup$
Another question I am struggling with:
Let $f:mathbb{R} rightarrow mathbb{R}$ be defined in a neighborhood of $a$ (except maybe $a$ itself) and suppose that, $lim_{x rightarrow a}left(f(x)+dfrac{1}{left|f(x)right|}right)=0$. Find $lim_{x rightarrow a}f(x)$, and prove by definition, that this is indeed the limit.
Thank you all for your answers!!
But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there?
calculus limits
$endgroup$
add a comment |
$begingroup$
Another question I am struggling with:
Let $f:mathbb{R} rightarrow mathbb{R}$ be defined in a neighborhood of $a$ (except maybe $a$ itself) and suppose that, $lim_{x rightarrow a}left(f(x)+dfrac{1}{left|f(x)right|}right)=0$. Find $lim_{x rightarrow a}f(x)$, and prove by definition, that this is indeed the limit.
Thank you all for your answers!!
But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there?
calculus limits
$endgroup$
Another question I am struggling with:
Let $f:mathbb{R} rightarrow mathbb{R}$ be defined in a neighborhood of $a$ (except maybe $a$ itself) and suppose that, $lim_{x rightarrow a}left(f(x)+dfrac{1}{left|f(x)right|}right)=0$. Find $lim_{x rightarrow a}f(x)$, and prove by definition, that this is indeed the limit.
Thank you all for your answers!!
But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there?
calculus limits
calculus limits
edited Jan 13 at 13:30
Did
248k23225464
248k23225464
asked Nov 30 '13 at 11:27
user83081user83081
41428
41428
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2 Answers
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$begingroup$
If $f(x)$ tends to a limit as $x to a$ then this limit must be $-1$. First we note that the limit $lim_{x to a}f(x)$, if it exists, must be negative. If the limit were positive then both $f(x)$ and $1/|f(x)|$ would have to be positive as $x to a$ and since their sum tends to $0$ each must tend to $0$ which is impossible. Similarly the limit can't be $0$ otherwise $1/|f(x)|$ would tend to $infty$. Now if we have the limit as $L < 0$, then $|f(x)| to -L$ and we get $L - (1/L) = 0$. This is possible only if $L = -1$.
$endgroup$
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
$begingroup$
Graphing the auxiliary function $$phi:quad umapsto u+{1over |u|}qquad(uindot{mathbb R})$$
we see that (a) $ |phi(u)|geq{5over6}$ when $uleq-{3over2}$ or $ugeq-{1over2}$, and that (b) the function $phi$ is continuous and strictly increasing on the interval $I: -{3over2}<u<-{1over2}$. From (b) we conclude that $phirestriction I$ has an inverse $phi^{-1}$ which is continuous on $phi(I)$.
Now we are told that the function
$$g(x):=f(x)+{1over |f(x)|}=phibigl(f(x)bigr)$$
has limit $0$ when $xto a$. It follows that there is a neighborhood $U$ of $a$ such that $|g(x)|<{5over6}$ for all $xindot U$. According to (a) above this enforces $-{3over2}<f(x)<-{1over2}$ for all $xindot U$. It follows that $f(x)=phi^{-1}bigl(g(x)bigr)$ for all $xindot U$, so that we obtain
$$lim_{xto a}f(x)=lim_{xto a}left(phi^{-1}bigl(g(x)bigr)right)=phi^{-1}(0)=-1 .$$
$endgroup$
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
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2 Answers
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2 Answers
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$begingroup$
If $f(x)$ tends to a limit as $x to a$ then this limit must be $-1$. First we note that the limit $lim_{x to a}f(x)$, if it exists, must be negative. If the limit were positive then both $f(x)$ and $1/|f(x)|$ would have to be positive as $x to a$ and since their sum tends to $0$ each must tend to $0$ which is impossible. Similarly the limit can't be $0$ otherwise $1/|f(x)|$ would tend to $infty$. Now if we have the limit as $L < 0$, then $|f(x)| to -L$ and we get $L - (1/L) = 0$. This is possible only if $L = -1$.
$endgroup$
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
$begingroup$
If $f(x)$ tends to a limit as $x to a$ then this limit must be $-1$. First we note that the limit $lim_{x to a}f(x)$, if it exists, must be negative. If the limit were positive then both $f(x)$ and $1/|f(x)|$ would have to be positive as $x to a$ and since their sum tends to $0$ each must tend to $0$ which is impossible. Similarly the limit can't be $0$ otherwise $1/|f(x)|$ would tend to $infty$. Now if we have the limit as $L < 0$, then $|f(x)| to -L$ and we get $L - (1/L) = 0$. This is possible only if $L = -1$.
$endgroup$
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
$begingroup$
If $f(x)$ tends to a limit as $x to a$ then this limit must be $-1$. First we note that the limit $lim_{x to a}f(x)$, if it exists, must be negative. If the limit were positive then both $f(x)$ and $1/|f(x)|$ would have to be positive as $x to a$ and since their sum tends to $0$ each must tend to $0$ which is impossible. Similarly the limit can't be $0$ otherwise $1/|f(x)|$ would tend to $infty$. Now if we have the limit as $L < 0$, then $|f(x)| to -L$ and we get $L - (1/L) = 0$. This is possible only if $L = -1$.
$endgroup$
If $f(x)$ tends to a limit as $x to a$ then this limit must be $-1$. First we note that the limit $lim_{x to a}f(x)$, if it exists, must be negative. If the limit were positive then both $f(x)$ and $1/|f(x)|$ would have to be positive as $x to a$ and since their sum tends to $0$ each must tend to $0$ which is impossible. Similarly the limit can't be $0$ otherwise $1/|f(x)|$ would tend to $infty$. Now if we have the limit as $L < 0$, then $|f(x)| to -L$ and we get $L - (1/L) = 0$. This is possible only if $L = -1$.
edited Nov 30 '13 at 12:02
answered Nov 30 '13 at 11:38


Paramanand SinghParamanand Singh
50.8k557168
50.8k557168
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
$begingroup$
Graphing the auxiliary function $$phi:quad umapsto u+{1over |u|}qquad(uindot{mathbb R})$$
we see that (a) $ |phi(u)|geq{5over6}$ when $uleq-{3over2}$ or $ugeq-{1over2}$, and that (b) the function $phi$ is continuous and strictly increasing on the interval $I: -{3over2}<u<-{1over2}$. From (b) we conclude that $phirestriction I$ has an inverse $phi^{-1}$ which is continuous on $phi(I)$.
Now we are told that the function
$$g(x):=f(x)+{1over |f(x)|}=phibigl(f(x)bigr)$$
has limit $0$ when $xto a$. It follows that there is a neighborhood $U$ of $a$ such that $|g(x)|<{5over6}$ for all $xindot U$. According to (a) above this enforces $-{3over2}<f(x)<-{1over2}$ for all $xindot U$. It follows that $f(x)=phi^{-1}bigl(g(x)bigr)$ for all $xindot U$, so that we obtain
$$lim_{xto a}f(x)=lim_{xto a}left(phi^{-1}bigl(g(x)bigr)right)=phi^{-1}(0)=-1 .$$
$endgroup$
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
$begingroup$
Graphing the auxiliary function $$phi:quad umapsto u+{1over |u|}qquad(uindot{mathbb R})$$
we see that (a) $ |phi(u)|geq{5over6}$ when $uleq-{3over2}$ or $ugeq-{1over2}$, and that (b) the function $phi$ is continuous and strictly increasing on the interval $I: -{3over2}<u<-{1over2}$. From (b) we conclude that $phirestriction I$ has an inverse $phi^{-1}$ which is continuous on $phi(I)$.
Now we are told that the function
$$g(x):=f(x)+{1over |f(x)|}=phibigl(f(x)bigr)$$
has limit $0$ when $xto a$. It follows that there is a neighborhood $U$ of $a$ such that $|g(x)|<{5over6}$ for all $xindot U$. According to (a) above this enforces $-{3over2}<f(x)<-{1over2}$ for all $xindot U$. It follows that $f(x)=phi^{-1}bigl(g(x)bigr)$ for all $xindot U$, so that we obtain
$$lim_{xto a}f(x)=lim_{xto a}left(phi^{-1}bigl(g(x)bigr)right)=phi^{-1}(0)=-1 .$$
$endgroup$
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
$begingroup$
Graphing the auxiliary function $$phi:quad umapsto u+{1over |u|}qquad(uindot{mathbb R})$$
we see that (a) $ |phi(u)|geq{5over6}$ when $uleq-{3over2}$ or $ugeq-{1over2}$, and that (b) the function $phi$ is continuous and strictly increasing on the interval $I: -{3over2}<u<-{1over2}$. From (b) we conclude that $phirestriction I$ has an inverse $phi^{-1}$ which is continuous on $phi(I)$.
Now we are told that the function
$$g(x):=f(x)+{1over |f(x)|}=phibigl(f(x)bigr)$$
has limit $0$ when $xto a$. It follows that there is a neighborhood $U$ of $a$ such that $|g(x)|<{5over6}$ for all $xindot U$. According to (a) above this enforces $-{3over2}<f(x)<-{1over2}$ for all $xindot U$. It follows that $f(x)=phi^{-1}bigl(g(x)bigr)$ for all $xindot U$, so that we obtain
$$lim_{xto a}f(x)=lim_{xto a}left(phi^{-1}bigl(g(x)bigr)right)=phi^{-1}(0)=-1 .$$
$endgroup$
Graphing the auxiliary function $$phi:quad umapsto u+{1over |u|}qquad(uindot{mathbb R})$$
we see that (a) $ |phi(u)|geq{5over6}$ when $uleq-{3over2}$ or $ugeq-{1over2}$, and that (b) the function $phi$ is continuous and strictly increasing on the interval $I: -{3over2}<u<-{1over2}$. From (b) we conclude that $phirestriction I$ has an inverse $phi^{-1}$ which is continuous on $phi(I)$.
Now we are told that the function
$$g(x):=f(x)+{1over |f(x)|}=phibigl(f(x)bigr)$$
has limit $0$ when $xto a$. It follows that there is a neighborhood $U$ of $a$ such that $|g(x)|<{5over6}$ for all $xindot U$. According to (a) above this enforces $-{3over2}<f(x)<-{1over2}$ for all $xindot U$. It follows that $f(x)=phi^{-1}bigl(g(x)bigr)$ for all $xindot U$, so that we obtain
$$lim_{xto a}f(x)=lim_{xto a}left(phi^{-1}bigl(g(x)bigr)right)=phi^{-1}(0)=-1 .$$
edited Nov 30 '13 at 13:40
answered Nov 30 '13 at 12:46


Christian BlatterChristian Blatter
175k8115327
175k8115327
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
$begingroup$
Thank you for your answer! But.. How can I prove by definition that this is indeed the limit? take $epsilon>0$, I have to show that there exists a $delta>0$ such that, $|x-a| leq delta$ implies $|f(x)+1| leq epsilon$. now, I do know that there exists an $delta_1>0$, such that $|f(x)+frac{1}{|f(x)|}|< epsilon$.Or, $frac{epsilon}{k}$ for some constant $k$ But, how can I continue from there? Thanks!
$endgroup$
– user83081
Nov 30 '13 at 15:47
add a comment |
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