without L'Hospital's rule find the $lim_{xto 0+} (csc x)^{sin^2 x}$
$begingroup$
$lim_{xto 0+}(csc x)^{sin^{2}x}$
Without using L'Hospital's rule, I've managed to get do:
Let $y=(csc x)^{sin^{2}x}$ now by taking $ln$ of both sides I got
$lim_{xto 0+}y$=$lim_{xto 0+}sin^{2}xln(csc x)$. Now by using the limit rules of a product if $lim_{xto 0+}sin^{2}x=T$ and the $lim_{xto 0+}ln(csc x)=H$ then $lim_{xto 0+}(csc x)^{sin^{2}x}=TH$
$lim_{xto 0+}sin^{2}x=0$ by algebra of limits, so the overall limit should either be indeterminate form or $0$ since $0.H=0$ if H is a non-indeterminate form
Knowing this I used the epsilon-delta definition to prove $lim_{xto 0+}ln(csc x)=0$
Let $epsilon$>$0$ such that
$|ln(csc x)-0|<epsilon$. I can ensure that $ln(csc x)<epsilon$ by requiring $x<sin^{-1}$ $frac{1}{e^{epsilon}}$. Thus:
I let $delta=$ $sin^{-1}$ $frac{1}{e^{epsilon}}$. And I think the conditions of the definition were satisfied. There the $lim_{xto 0+}ln(csc x)=0$
Thus $lim_{xto 0+}(csc x)^{sin^{2}x}=0$
My question is where have I gone wrong because the answer is 1, but I cannot find my own fault.
Thank you in advance,.
real-analysis limits limits-without-lhopital
$endgroup$
add a comment |
$begingroup$
$lim_{xto 0+}(csc x)^{sin^{2}x}$
Without using L'Hospital's rule, I've managed to get do:
Let $y=(csc x)^{sin^{2}x}$ now by taking $ln$ of both sides I got
$lim_{xto 0+}y$=$lim_{xto 0+}sin^{2}xln(csc x)$. Now by using the limit rules of a product if $lim_{xto 0+}sin^{2}x=T$ and the $lim_{xto 0+}ln(csc x)=H$ then $lim_{xto 0+}(csc x)^{sin^{2}x}=TH$
$lim_{xto 0+}sin^{2}x=0$ by algebra of limits, so the overall limit should either be indeterminate form or $0$ since $0.H=0$ if H is a non-indeterminate form
Knowing this I used the epsilon-delta definition to prove $lim_{xto 0+}ln(csc x)=0$
Let $epsilon$>$0$ such that
$|ln(csc x)-0|<epsilon$. I can ensure that $ln(csc x)<epsilon$ by requiring $x<sin^{-1}$ $frac{1}{e^{epsilon}}$. Thus:
I let $delta=$ $sin^{-1}$ $frac{1}{e^{epsilon}}$. And I think the conditions of the definition were satisfied. There the $lim_{xto 0+}ln(csc x)=0$
Thus $lim_{xto 0+}(csc x)^{sin^{2}x}=0$
My question is where have I gone wrong because the answer is 1, but I cannot find my own fault.
Thank you in advance,.
real-analysis limits limits-without-lhopital
$endgroup$
1
$begingroup$
You computed$ lim ln(y).$
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 20:43
2
$begingroup$
An obvious start would be to switch variable to $u=sin x$ -- then you're looking for $lim_{uto 0^+} u^{-u^2}$ instead and need not trouble yourself with more trigonometry.
$endgroup$
– Henning Makholm
Dec 23 '18 at 20:52
$begingroup$
@ricky If $ln y$ goes to zero, it means that $y$ goes to one.
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 21:04
$begingroup$
could you expand further I'm struggling to see how
$endgroup$
– ricky
Dec 23 '18 at 21:06
1
$begingroup$
But $limlimits_{xto 0^+}ln(sin x) = -infty$, so $limlimits_{xto 0^+}ln(csc x) = infty$
$endgroup$
– Ted Shifrin
Dec 23 '18 at 21:21
add a comment |
$begingroup$
$lim_{xto 0+}(csc x)^{sin^{2}x}$
Without using L'Hospital's rule, I've managed to get do:
Let $y=(csc x)^{sin^{2}x}$ now by taking $ln$ of both sides I got
$lim_{xto 0+}y$=$lim_{xto 0+}sin^{2}xln(csc x)$. Now by using the limit rules of a product if $lim_{xto 0+}sin^{2}x=T$ and the $lim_{xto 0+}ln(csc x)=H$ then $lim_{xto 0+}(csc x)^{sin^{2}x}=TH$
$lim_{xto 0+}sin^{2}x=0$ by algebra of limits, so the overall limit should either be indeterminate form or $0$ since $0.H=0$ if H is a non-indeterminate form
Knowing this I used the epsilon-delta definition to prove $lim_{xto 0+}ln(csc x)=0$
Let $epsilon$>$0$ such that
$|ln(csc x)-0|<epsilon$. I can ensure that $ln(csc x)<epsilon$ by requiring $x<sin^{-1}$ $frac{1}{e^{epsilon}}$. Thus:
I let $delta=$ $sin^{-1}$ $frac{1}{e^{epsilon}}$. And I think the conditions of the definition were satisfied. There the $lim_{xto 0+}ln(csc x)=0$
Thus $lim_{xto 0+}(csc x)^{sin^{2}x}=0$
My question is where have I gone wrong because the answer is 1, but I cannot find my own fault.
Thank you in advance,.
real-analysis limits limits-without-lhopital
$endgroup$
$lim_{xto 0+}(csc x)^{sin^{2}x}$
Without using L'Hospital's rule, I've managed to get do:
Let $y=(csc x)^{sin^{2}x}$ now by taking $ln$ of both sides I got
$lim_{xto 0+}y$=$lim_{xto 0+}sin^{2}xln(csc x)$. Now by using the limit rules of a product if $lim_{xto 0+}sin^{2}x=T$ and the $lim_{xto 0+}ln(csc x)=H$ then $lim_{xto 0+}(csc x)^{sin^{2}x}=TH$
$lim_{xto 0+}sin^{2}x=0$ by algebra of limits, so the overall limit should either be indeterminate form or $0$ since $0.H=0$ if H is a non-indeterminate form
Knowing this I used the epsilon-delta definition to prove $lim_{xto 0+}ln(csc x)=0$
Let $epsilon$>$0$ such that
$|ln(csc x)-0|<epsilon$. I can ensure that $ln(csc x)<epsilon$ by requiring $x<sin^{-1}$ $frac{1}{e^{epsilon}}$. Thus:
I let $delta=$ $sin^{-1}$ $frac{1}{e^{epsilon}}$. And I think the conditions of the definition were satisfied. There the $lim_{xto 0+}ln(csc x)=0$
Thus $lim_{xto 0+}(csc x)^{sin^{2}x}=0$
My question is where have I gone wrong because the answer is 1, but I cannot find my own fault.
Thank you in advance,.
real-analysis limits limits-without-lhopital
real-analysis limits limits-without-lhopital
edited Dec 23 '18 at 20:56
Bernard
124k742117
124k742117
asked Dec 23 '18 at 20:41
rickyricky
1337
1337
1
$begingroup$
You computed$ lim ln(y).$
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 20:43
2
$begingroup$
An obvious start would be to switch variable to $u=sin x$ -- then you're looking for $lim_{uto 0^+} u^{-u^2}$ instead and need not trouble yourself with more trigonometry.
$endgroup$
– Henning Makholm
Dec 23 '18 at 20:52
$begingroup$
@ricky If $ln y$ goes to zero, it means that $y$ goes to one.
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 21:04
$begingroup$
could you expand further I'm struggling to see how
$endgroup$
– ricky
Dec 23 '18 at 21:06
1
$begingroup$
But $limlimits_{xto 0^+}ln(sin x) = -infty$, so $limlimits_{xto 0^+}ln(csc x) = infty$
$endgroup$
– Ted Shifrin
Dec 23 '18 at 21:21
add a comment |
1
$begingroup$
You computed$ lim ln(y).$
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 20:43
2
$begingroup$
An obvious start would be to switch variable to $u=sin x$ -- then you're looking for $lim_{uto 0^+} u^{-u^2}$ instead and need not trouble yourself with more trigonometry.
$endgroup$
– Henning Makholm
Dec 23 '18 at 20:52
$begingroup$
@ricky If $ln y$ goes to zero, it means that $y$ goes to one.
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 21:04
$begingroup$
could you expand further I'm struggling to see how
$endgroup$
– ricky
Dec 23 '18 at 21:06
1
$begingroup$
But $limlimits_{xto 0^+}ln(sin x) = -infty$, so $limlimits_{xto 0^+}ln(csc x) = infty$
$endgroup$
– Ted Shifrin
Dec 23 '18 at 21:21
1
1
$begingroup$
You computed$ lim ln(y).$
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 20:43
$begingroup$
You computed$ lim ln(y).$
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 20:43
2
2
$begingroup$
An obvious start would be to switch variable to $u=sin x$ -- then you're looking for $lim_{uto 0^+} u^{-u^2}$ instead and need not trouble yourself with more trigonometry.
$endgroup$
– Henning Makholm
Dec 23 '18 at 20:52
$begingroup$
An obvious start would be to switch variable to $u=sin x$ -- then you're looking for $lim_{uto 0^+} u^{-u^2}$ instead and need not trouble yourself with more trigonometry.
$endgroup$
– Henning Makholm
Dec 23 '18 at 20:52
$begingroup$
@ricky If $ln y$ goes to zero, it means that $y$ goes to one.
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 21:04
$begingroup$
@ricky If $ln y$ goes to zero, it means that $y$ goes to one.
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 21:04
$begingroup$
could you expand further I'm struggling to see how
$endgroup$
– ricky
Dec 23 '18 at 21:06
$begingroup$
could you expand further I'm struggling to see how
$endgroup$
– ricky
Dec 23 '18 at 21:06
1
1
$begingroup$
But $limlimits_{xto 0^+}ln(sin x) = -infty$, so $limlimits_{xto 0^+}ln(csc x) = infty$
$endgroup$
– Ted Shifrin
Dec 23 '18 at 21:21
$begingroup$
But $limlimits_{xto 0^+}ln(sin x) = -infty$, so $limlimits_{xto 0^+}ln(csc x) = infty$
$endgroup$
– Ted Shifrin
Dec 23 '18 at 21:21
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
No need to use $varepsilon$-$delta$. See also at the end for other comments.
Note first that $lncsc x=-lnsin x$ so you have to compute
$$
lim_{xto0}-sin^2x lnsin x
$$
Now use the substitution $x=arcsin t$, so the limit becomes
$$
lim_{tto 0}-t^2ln t
$$
which is known to be $0$. Since
$$
lim_{xto0^+}lnbigl((csc x)^{sin^2x}bigr)=0
$$
you can conclude that
$$
lim_{xto0^+}(csc x)^{sin^2x}=1
$$
Note that
$$
lim_{xto0}lncsc x=infty
$$
and certainly not $0$ as you claim. So, even if you found the correct limit $0$, your method is flawed.
$endgroup$
$begingroup$
Thank you, made it look so simple haha
$endgroup$
– ricky
Dec 23 '18 at 23:54
add a comment |
$begingroup$
Let's see where you went wrong. We know that
$$lim_{xto 0^+}sin(x)=0$$
$$Longrightarrow lim_{xto 0^+}cosec(x)=+infty$$
$$(because sin(x)>0 forall xin(0,π))$$
$$Longrightarrow lim_{xto 0^+}ln(cosec(x))=+infty$$
And you claimed that to be $0$.
Now, coming to the question, let the required limit be $y$.
$$Longrightarrow y=lim_{xto 0^+}(cosec(x))^{sin^2(x)}$$
$$Longrightarrow ln(y)=lim_{xto 0^+}(sin^2(x))ln(cosec(x))$$
As $xto 0^+,cosec(x)to infty$. Hence,
$$Longrightarrow ln(y)=lim_{uto infty}frac{ln(u)}{u^2}$$
$$Longrightarrow ln(y)=lim_{utoinfty}frac{ln(u)}{u}×lim_{utoinfty}frac{1}{u}$$
both of which limits are clearly $0$.
$$Longrightarrow ln(y)=0×0=0$$
$$Longrightarrow y=1$$
which is the required answer.
Hope it helps
$endgroup$
$begingroup$
Thank you, I would of verified your answer but I can only choose one
$endgroup$
– ricky
Dec 23 '18 at 23:55
add a comment |
Your Answer
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2 Answers
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active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
No need to use $varepsilon$-$delta$. See also at the end for other comments.
Note first that $lncsc x=-lnsin x$ so you have to compute
$$
lim_{xto0}-sin^2x lnsin x
$$
Now use the substitution $x=arcsin t$, so the limit becomes
$$
lim_{tto 0}-t^2ln t
$$
which is known to be $0$. Since
$$
lim_{xto0^+}lnbigl((csc x)^{sin^2x}bigr)=0
$$
you can conclude that
$$
lim_{xto0^+}(csc x)^{sin^2x}=1
$$
Note that
$$
lim_{xto0}lncsc x=infty
$$
and certainly not $0$ as you claim. So, even if you found the correct limit $0$, your method is flawed.
$endgroup$
$begingroup$
Thank you, made it look so simple haha
$endgroup$
– ricky
Dec 23 '18 at 23:54
add a comment |
$begingroup$
No need to use $varepsilon$-$delta$. See also at the end for other comments.
Note first that $lncsc x=-lnsin x$ so you have to compute
$$
lim_{xto0}-sin^2x lnsin x
$$
Now use the substitution $x=arcsin t$, so the limit becomes
$$
lim_{tto 0}-t^2ln t
$$
which is known to be $0$. Since
$$
lim_{xto0^+}lnbigl((csc x)^{sin^2x}bigr)=0
$$
you can conclude that
$$
lim_{xto0^+}(csc x)^{sin^2x}=1
$$
Note that
$$
lim_{xto0}lncsc x=infty
$$
and certainly not $0$ as you claim. So, even if you found the correct limit $0$, your method is flawed.
$endgroup$
$begingroup$
Thank you, made it look so simple haha
$endgroup$
– ricky
Dec 23 '18 at 23:54
add a comment |
$begingroup$
No need to use $varepsilon$-$delta$. See also at the end for other comments.
Note first that $lncsc x=-lnsin x$ so you have to compute
$$
lim_{xto0}-sin^2x lnsin x
$$
Now use the substitution $x=arcsin t$, so the limit becomes
$$
lim_{tto 0}-t^2ln t
$$
which is known to be $0$. Since
$$
lim_{xto0^+}lnbigl((csc x)^{sin^2x}bigr)=0
$$
you can conclude that
$$
lim_{xto0^+}(csc x)^{sin^2x}=1
$$
Note that
$$
lim_{xto0}lncsc x=infty
$$
and certainly not $0$ as you claim. So, even if you found the correct limit $0$, your method is flawed.
$endgroup$
No need to use $varepsilon$-$delta$. See also at the end for other comments.
Note first that $lncsc x=-lnsin x$ so you have to compute
$$
lim_{xto0}-sin^2x lnsin x
$$
Now use the substitution $x=arcsin t$, so the limit becomes
$$
lim_{tto 0}-t^2ln t
$$
which is known to be $0$. Since
$$
lim_{xto0^+}lnbigl((csc x)^{sin^2x}bigr)=0
$$
you can conclude that
$$
lim_{xto0^+}(csc x)^{sin^2x}=1
$$
Note that
$$
lim_{xto0}lncsc x=infty
$$
and certainly not $0$ as you claim. So, even if you found the correct limit $0$, your method is flawed.
answered Dec 23 '18 at 23:46
egregegreg
186k1486209
186k1486209
$begingroup$
Thank you, made it look so simple haha
$endgroup$
– ricky
Dec 23 '18 at 23:54
add a comment |
$begingroup$
Thank you, made it look so simple haha
$endgroup$
– ricky
Dec 23 '18 at 23:54
$begingroup$
Thank you, made it look so simple haha
$endgroup$
– ricky
Dec 23 '18 at 23:54
$begingroup$
Thank you, made it look so simple haha
$endgroup$
– ricky
Dec 23 '18 at 23:54
add a comment |
$begingroup$
Let's see where you went wrong. We know that
$$lim_{xto 0^+}sin(x)=0$$
$$Longrightarrow lim_{xto 0^+}cosec(x)=+infty$$
$$(because sin(x)>0 forall xin(0,π))$$
$$Longrightarrow lim_{xto 0^+}ln(cosec(x))=+infty$$
And you claimed that to be $0$.
Now, coming to the question, let the required limit be $y$.
$$Longrightarrow y=lim_{xto 0^+}(cosec(x))^{sin^2(x)}$$
$$Longrightarrow ln(y)=lim_{xto 0^+}(sin^2(x))ln(cosec(x))$$
As $xto 0^+,cosec(x)to infty$. Hence,
$$Longrightarrow ln(y)=lim_{uto infty}frac{ln(u)}{u^2}$$
$$Longrightarrow ln(y)=lim_{utoinfty}frac{ln(u)}{u}×lim_{utoinfty}frac{1}{u}$$
both of which limits are clearly $0$.
$$Longrightarrow ln(y)=0×0=0$$
$$Longrightarrow y=1$$
which is the required answer.
Hope it helps
$endgroup$
$begingroup$
Thank you, I would of verified your answer but I can only choose one
$endgroup$
– ricky
Dec 23 '18 at 23:55
add a comment |
$begingroup$
Let's see where you went wrong. We know that
$$lim_{xto 0^+}sin(x)=0$$
$$Longrightarrow lim_{xto 0^+}cosec(x)=+infty$$
$$(because sin(x)>0 forall xin(0,π))$$
$$Longrightarrow lim_{xto 0^+}ln(cosec(x))=+infty$$
And you claimed that to be $0$.
Now, coming to the question, let the required limit be $y$.
$$Longrightarrow y=lim_{xto 0^+}(cosec(x))^{sin^2(x)}$$
$$Longrightarrow ln(y)=lim_{xto 0^+}(sin^2(x))ln(cosec(x))$$
As $xto 0^+,cosec(x)to infty$. Hence,
$$Longrightarrow ln(y)=lim_{uto infty}frac{ln(u)}{u^2}$$
$$Longrightarrow ln(y)=lim_{utoinfty}frac{ln(u)}{u}×lim_{utoinfty}frac{1}{u}$$
both of which limits are clearly $0$.
$$Longrightarrow ln(y)=0×0=0$$
$$Longrightarrow y=1$$
which is the required answer.
Hope it helps
$endgroup$
$begingroup$
Thank you, I would of verified your answer but I can only choose one
$endgroup$
– ricky
Dec 23 '18 at 23:55
add a comment |
$begingroup$
Let's see where you went wrong. We know that
$$lim_{xto 0^+}sin(x)=0$$
$$Longrightarrow lim_{xto 0^+}cosec(x)=+infty$$
$$(because sin(x)>0 forall xin(0,π))$$
$$Longrightarrow lim_{xto 0^+}ln(cosec(x))=+infty$$
And you claimed that to be $0$.
Now, coming to the question, let the required limit be $y$.
$$Longrightarrow y=lim_{xto 0^+}(cosec(x))^{sin^2(x)}$$
$$Longrightarrow ln(y)=lim_{xto 0^+}(sin^2(x))ln(cosec(x))$$
As $xto 0^+,cosec(x)to infty$. Hence,
$$Longrightarrow ln(y)=lim_{uto infty}frac{ln(u)}{u^2}$$
$$Longrightarrow ln(y)=lim_{utoinfty}frac{ln(u)}{u}×lim_{utoinfty}frac{1}{u}$$
both of which limits are clearly $0$.
$$Longrightarrow ln(y)=0×0=0$$
$$Longrightarrow y=1$$
which is the required answer.
Hope it helps
$endgroup$
Let's see where you went wrong. We know that
$$lim_{xto 0^+}sin(x)=0$$
$$Longrightarrow lim_{xto 0^+}cosec(x)=+infty$$
$$(because sin(x)>0 forall xin(0,π))$$
$$Longrightarrow lim_{xto 0^+}ln(cosec(x))=+infty$$
And you claimed that to be $0$.
Now, coming to the question, let the required limit be $y$.
$$Longrightarrow y=lim_{xto 0^+}(cosec(x))^{sin^2(x)}$$
$$Longrightarrow ln(y)=lim_{xto 0^+}(sin^2(x))ln(cosec(x))$$
As $xto 0^+,cosec(x)to infty$. Hence,
$$Longrightarrow ln(y)=lim_{uto infty}frac{ln(u)}{u^2}$$
$$Longrightarrow ln(y)=lim_{utoinfty}frac{ln(u)}{u}×lim_{utoinfty}frac{1}{u}$$
both of which limits are clearly $0$.
$$Longrightarrow ln(y)=0×0=0$$
$$Longrightarrow y=1$$
which is the required answer.
Hope it helps
edited Dec 23 '18 at 23:57
answered Dec 23 '18 at 23:30
MartundMartund
2,1451213
2,1451213
$begingroup$
Thank you, I would of verified your answer but I can only choose one
$endgroup$
– ricky
Dec 23 '18 at 23:55
add a comment |
$begingroup$
Thank you, I would of verified your answer but I can only choose one
$endgroup$
– ricky
Dec 23 '18 at 23:55
$begingroup$
Thank you, I would of verified your answer but I can only choose one
$endgroup$
– ricky
Dec 23 '18 at 23:55
$begingroup$
Thank you, I would of verified your answer but I can only choose one
$endgroup$
– ricky
Dec 23 '18 at 23:55
add a comment |
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1
$begingroup$
You computed$ lim ln(y).$
$endgroup$
– hamam_Abdallah
Dec 23 '18 at 20:43
2
$begingroup$
An obvious start would be to switch variable to $u=sin x$ -- then you're looking for $lim_{uto 0^+} u^{-u^2}$ instead and need not trouble yourself with more trigonometry.
$endgroup$
– Henning Makholm
Dec 23 '18 at 20:52
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@ricky If $ln y$ goes to zero, it means that $y$ goes to one.
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– hamam_Abdallah
Dec 23 '18 at 21:04
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could you expand further I'm struggling to see how
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– ricky
Dec 23 '18 at 21:06
1
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But $limlimits_{xto 0^+}ln(sin x) = -infty$, so $limlimits_{xto 0^+}ln(csc x) = infty$
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– Ted Shifrin
Dec 23 '18 at 21:21