How to find the $lim_{ntoinfty}s_n$ when $s_1=5, s_n =sqrt{2+s_{n-1}}$?
$begingroup$
How to find the $lim_{ntoinfty}s_n$ when $s_1=5, s_n =sqrt{2+s_{n-1}}$ using the Monotone Convergence Theorem?
I have the proof from my professor, but I am stuck at one step.
Proof:
Step 1: Show that it is monotonic:
Proof by induction:
Claim: $s_n > s_{n+1}, forall ninmathbb{N}$
Base case: $n=1$. $s_1 = 5 > s_2=sqrt{7}$, so it holds for $n=1$.
Assume it holds for $n=k$, now show it's true for $n=k+1$.
Assume $s_k>s_{k+1}$, then $s_{(k+1)+1} = s_{k+2} = sqrt{2+s_{k+1}} < sqrt{2+s_k} = s_{k+1}$ by assumption. Therefore, by the Principle of Mathematical Induction, the statement is true $forall ninmathbb{N}$ and ${S_n}$ is decreasing (monotonic).
Step 2: Find the limit:
It is clear $0leq s_nleq s_1 = 5forall n$, so ${S_n}$ is bounded.
Therefore, since ${S_n}$ is monotone and bounded, by the Monotone Convergence Theorem, ${S_n}$ converges.
Let $lim_{ntoinfty} s_n = L$:
begin{align}
lim_{ntoinfty}sqrt{2+s_{n-1}} &= L\
sqrt{2 + lim_{ntoinfty} s_{n-1}} &= L\
sqrt{2+L} &= L *text{This is where I get stuck...}\
2+L &= L^2\
0 &= L^2 - L - 2\
&= (L-2)(L+1)\
&implies L=-1, 2
end{align}
But $Lne-1$ since $s_ngeq 0forall n$. Therefore, $lim_{ntoinfty} s_n = 2$.
I get stuck because I am unsure why we can say $lim_{ntoinfty} s_{n-1} = L$. We said that $lim_{ntoinfty} s_{n} = L$, but nothing about the limit of $s_{n-1}$.
real-analysis limits monotone-functions
$endgroup$
add a comment |
$begingroup$
How to find the $lim_{ntoinfty}s_n$ when $s_1=5, s_n =sqrt{2+s_{n-1}}$ using the Monotone Convergence Theorem?
I have the proof from my professor, but I am stuck at one step.
Proof:
Step 1: Show that it is monotonic:
Proof by induction:
Claim: $s_n > s_{n+1}, forall ninmathbb{N}$
Base case: $n=1$. $s_1 = 5 > s_2=sqrt{7}$, so it holds for $n=1$.
Assume it holds for $n=k$, now show it's true for $n=k+1$.
Assume $s_k>s_{k+1}$, then $s_{(k+1)+1} = s_{k+2} = sqrt{2+s_{k+1}} < sqrt{2+s_k} = s_{k+1}$ by assumption. Therefore, by the Principle of Mathematical Induction, the statement is true $forall ninmathbb{N}$ and ${S_n}$ is decreasing (monotonic).
Step 2: Find the limit:
It is clear $0leq s_nleq s_1 = 5forall n$, so ${S_n}$ is bounded.
Therefore, since ${S_n}$ is monotone and bounded, by the Monotone Convergence Theorem, ${S_n}$ converges.
Let $lim_{ntoinfty} s_n = L$:
begin{align}
lim_{ntoinfty}sqrt{2+s_{n-1}} &= L\
sqrt{2 + lim_{ntoinfty} s_{n-1}} &= L\
sqrt{2+L} &= L *text{This is where I get stuck...}\
2+L &= L^2\
0 &= L^2 - L - 2\
&= (L-2)(L+1)\
&implies L=-1, 2
end{align}
But $Lne-1$ since $s_ngeq 0forall n$. Therefore, $lim_{ntoinfty} s_n = 2$.
I get stuck because I am unsure why we can say $lim_{ntoinfty} s_{n-1} = L$. We said that $lim_{ntoinfty} s_{n} = L$, but nothing about the limit of $s_{n-1}$.
real-analysis limits monotone-functions
$endgroup$
1
$begingroup$
It is the tail that matters. The tail of $s_{n-1}$ and $s_n$ are the same. So they converge to the same thing.
$endgroup$
– user9077
Dec 15 '18 at 15:55
1
$begingroup$
What happens to $n-1$ as $ntoinfty?$
$endgroup$
– saulspatz
Dec 15 '18 at 15:55
add a comment |
$begingroup$
How to find the $lim_{ntoinfty}s_n$ when $s_1=5, s_n =sqrt{2+s_{n-1}}$ using the Monotone Convergence Theorem?
I have the proof from my professor, but I am stuck at one step.
Proof:
Step 1: Show that it is monotonic:
Proof by induction:
Claim: $s_n > s_{n+1}, forall ninmathbb{N}$
Base case: $n=1$. $s_1 = 5 > s_2=sqrt{7}$, so it holds for $n=1$.
Assume it holds for $n=k$, now show it's true for $n=k+1$.
Assume $s_k>s_{k+1}$, then $s_{(k+1)+1} = s_{k+2} = sqrt{2+s_{k+1}} < sqrt{2+s_k} = s_{k+1}$ by assumption. Therefore, by the Principle of Mathematical Induction, the statement is true $forall ninmathbb{N}$ and ${S_n}$ is decreasing (monotonic).
Step 2: Find the limit:
It is clear $0leq s_nleq s_1 = 5forall n$, so ${S_n}$ is bounded.
Therefore, since ${S_n}$ is monotone and bounded, by the Monotone Convergence Theorem, ${S_n}$ converges.
Let $lim_{ntoinfty} s_n = L$:
begin{align}
lim_{ntoinfty}sqrt{2+s_{n-1}} &= L\
sqrt{2 + lim_{ntoinfty} s_{n-1}} &= L\
sqrt{2+L} &= L *text{This is where I get stuck...}\
2+L &= L^2\
0 &= L^2 - L - 2\
&= (L-2)(L+1)\
&implies L=-1, 2
end{align}
But $Lne-1$ since $s_ngeq 0forall n$. Therefore, $lim_{ntoinfty} s_n = 2$.
I get stuck because I am unsure why we can say $lim_{ntoinfty} s_{n-1} = L$. We said that $lim_{ntoinfty} s_{n} = L$, but nothing about the limit of $s_{n-1}$.
real-analysis limits monotone-functions
$endgroup$
How to find the $lim_{ntoinfty}s_n$ when $s_1=5, s_n =sqrt{2+s_{n-1}}$ using the Monotone Convergence Theorem?
I have the proof from my professor, but I am stuck at one step.
Proof:
Step 1: Show that it is monotonic:
Proof by induction:
Claim: $s_n > s_{n+1}, forall ninmathbb{N}$
Base case: $n=1$. $s_1 = 5 > s_2=sqrt{7}$, so it holds for $n=1$.
Assume it holds for $n=k$, now show it's true for $n=k+1$.
Assume $s_k>s_{k+1}$, then $s_{(k+1)+1} = s_{k+2} = sqrt{2+s_{k+1}} < sqrt{2+s_k} = s_{k+1}$ by assumption. Therefore, by the Principle of Mathematical Induction, the statement is true $forall ninmathbb{N}$ and ${S_n}$ is decreasing (monotonic).
Step 2: Find the limit:
It is clear $0leq s_nleq s_1 = 5forall n$, so ${S_n}$ is bounded.
Therefore, since ${S_n}$ is monotone and bounded, by the Monotone Convergence Theorem, ${S_n}$ converges.
Let $lim_{ntoinfty} s_n = L$:
begin{align}
lim_{ntoinfty}sqrt{2+s_{n-1}} &= L\
sqrt{2 + lim_{ntoinfty} s_{n-1}} &= L\
sqrt{2+L} &= L *text{This is where I get stuck...}\
2+L &= L^2\
0 &= L^2 - L - 2\
&= (L-2)(L+1)\
&implies L=-1, 2
end{align}
But $Lne-1$ since $s_ngeq 0forall n$. Therefore, $lim_{ntoinfty} s_n = 2$.
I get stuck because I am unsure why we can say $lim_{ntoinfty} s_{n-1} = L$. We said that $lim_{ntoinfty} s_{n} = L$, but nothing about the limit of $s_{n-1}$.
real-analysis limits monotone-functions
real-analysis limits monotone-functions
edited Dec 15 '18 at 15:58
Bernard
121k740116
121k740116
asked Dec 15 '18 at 15:50
kaisakaisa
2019
2019
1
$begingroup$
It is the tail that matters. The tail of $s_{n-1}$ and $s_n$ are the same. So they converge to the same thing.
$endgroup$
– user9077
Dec 15 '18 at 15:55
1
$begingroup$
What happens to $n-1$ as $ntoinfty?$
$endgroup$
– saulspatz
Dec 15 '18 at 15:55
add a comment |
1
$begingroup$
It is the tail that matters. The tail of $s_{n-1}$ and $s_n$ are the same. So they converge to the same thing.
$endgroup$
– user9077
Dec 15 '18 at 15:55
1
$begingroup$
What happens to $n-1$ as $ntoinfty?$
$endgroup$
– saulspatz
Dec 15 '18 at 15:55
1
1
$begingroup$
It is the tail that matters. The tail of $s_{n-1}$ and $s_n$ are the same. So they converge to the same thing.
$endgroup$
– user9077
Dec 15 '18 at 15:55
$begingroup$
It is the tail that matters. The tail of $s_{n-1}$ and $s_n$ are the same. So they converge to the same thing.
$endgroup$
– user9077
Dec 15 '18 at 15:55
1
1
$begingroup$
What happens to $n-1$ as $ntoinfty?$
$endgroup$
– saulspatz
Dec 15 '18 at 15:55
$begingroup$
What happens to $n-1$ as $ntoinfty?$
$endgroup$
– saulspatz
Dec 15 '18 at 15:55
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Write down the definition of a limit to show that if $(a_n)$ is any converging sequence, then $(a_{n-1})$ is also a converging sequence and both limits are the same.
$endgroup$
add a comment |
$begingroup$
The sequence is decreasing, because $s_{n+1}<s_n$ is equivalent to
$$
s_n+2<s_n^2
$$
which is satisfied when $s_n<-1$ or $s_n>2$. The latter is true by induction: $s_1=5>2$; suppose $s_k>2$; then $s_{k+1}=sqrt{s_k+2}>sqrt{2+2}=2$.
Your proof is good as well.
The sequence is bounded below by $0$, hence it converges. If $l$ is its limit, then $lge0$ and
$$
l=lim_{ntoinfty}s_n=lim_{ntoinfty}s_{n+1}=sqrt{l+2}
$$
Therefore $l=2$.
Actually, $lim_{ntoinfty}s_{n+1}$ should really be $lim_{ntoinfty}s'_n$, where
$$
s'_n=s_{n+1}
$$
It's essentially going through the definition of limit to show that $(s_n)$ converges if and only if $(s'_n)$ and, in this case, they have the same limit.
Since $s'_n=sqrt{s_n+1}$, the theorems on limits also allow us to say that $$l=lim_{ntoinfty}s'_n=lim_{ntoinfty}sqrt{s_n+2}=sqrt{l+2}$$
Note: I wouldn't use $s_{n-1}$, but it's just personal preference.
$endgroup$
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Write down the definition of a limit to show that if $(a_n)$ is any converging sequence, then $(a_{n-1})$ is also a converging sequence and both limits are the same.
$endgroup$
add a comment |
$begingroup$
Write down the definition of a limit to show that if $(a_n)$ is any converging sequence, then $(a_{n-1})$ is also a converging sequence and both limits are the same.
$endgroup$
add a comment |
$begingroup$
Write down the definition of a limit to show that if $(a_n)$ is any converging sequence, then $(a_{n-1})$ is also a converging sequence and both limits are the same.
$endgroup$
Write down the definition of a limit to show that if $(a_n)$ is any converging sequence, then $(a_{n-1})$ is also a converging sequence and both limits are the same.
answered Dec 15 '18 at 15:52
MindlackMindlack
4,450210
4,450210
add a comment |
add a comment |
$begingroup$
The sequence is decreasing, because $s_{n+1}<s_n$ is equivalent to
$$
s_n+2<s_n^2
$$
which is satisfied when $s_n<-1$ or $s_n>2$. The latter is true by induction: $s_1=5>2$; suppose $s_k>2$; then $s_{k+1}=sqrt{s_k+2}>sqrt{2+2}=2$.
Your proof is good as well.
The sequence is bounded below by $0$, hence it converges. If $l$ is its limit, then $lge0$ and
$$
l=lim_{ntoinfty}s_n=lim_{ntoinfty}s_{n+1}=sqrt{l+2}
$$
Therefore $l=2$.
Actually, $lim_{ntoinfty}s_{n+1}$ should really be $lim_{ntoinfty}s'_n$, where
$$
s'_n=s_{n+1}
$$
It's essentially going through the definition of limit to show that $(s_n)$ converges if and only if $(s'_n)$ and, in this case, they have the same limit.
Since $s'_n=sqrt{s_n+1}$, the theorems on limits also allow us to say that $$l=lim_{ntoinfty}s'_n=lim_{ntoinfty}sqrt{s_n+2}=sqrt{l+2}$$
Note: I wouldn't use $s_{n-1}$, but it's just personal preference.
$endgroup$
add a comment |
$begingroup$
The sequence is decreasing, because $s_{n+1}<s_n$ is equivalent to
$$
s_n+2<s_n^2
$$
which is satisfied when $s_n<-1$ or $s_n>2$. The latter is true by induction: $s_1=5>2$; suppose $s_k>2$; then $s_{k+1}=sqrt{s_k+2}>sqrt{2+2}=2$.
Your proof is good as well.
The sequence is bounded below by $0$, hence it converges. If $l$ is its limit, then $lge0$ and
$$
l=lim_{ntoinfty}s_n=lim_{ntoinfty}s_{n+1}=sqrt{l+2}
$$
Therefore $l=2$.
Actually, $lim_{ntoinfty}s_{n+1}$ should really be $lim_{ntoinfty}s'_n$, where
$$
s'_n=s_{n+1}
$$
It's essentially going through the definition of limit to show that $(s_n)$ converges if and only if $(s'_n)$ and, in this case, they have the same limit.
Since $s'_n=sqrt{s_n+1}$, the theorems on limits also allow us to say that $$l=lim_{ntoinfty}s'_n=lim_{ntoinfty}sqrt{s_n+2}=sqrt{l+2}$$
Note: I wouldn't use $s_{n-1}$, but it's just personal preference.
$endgroup$
add a comment |
$begingroup$
The sequence is decreasing, because $s_{n+1}<s_n$ is equivalent to
$$
s_n+2<s_n^2
$$
which is satisfied when $s_n<-1$ or $s_n>2$. The latter is true by induction: $s_1=5>2$; suppose $s_k>2$; then $s_{k+1}=sqrt{s_k+2}>sqrt{2+2}=2$.
Your proof is good as well.
The sequence is bounded below by $0$, hence it converges. If $l$ is its limit, then $lge0$ and
$$
l=lim_{ntoinfty}s_n=lim_{ntoinfty}s_{n+1}=sqrt{l+2}
$$
Therefore $l=2$.
Actually, $lim_{ntoinfty}s_{n+1}$ should really be $lim_{ntoinfty}s'_n$, where
$$
s'_n=s_{n+1}
$$
It's essentially going through the definition of limit to show that $(s_n)$ converges if and only if $(s'_n)$ and, in this case, they have the same limit.
Since $s'_n=sqrt{s_n+1}$, the theorems on limits also allow us to say that $$l=lim_{ntoinfty}s'_n=lim_{ntoinfty}sqrt{s_n+2}=sqrt{l+2}$$
Note: I wouldn't use $s_{n-1}$, but it's just personal preference.
$endgroup$
The sequence is decreasing, because $s_{n+1}<s_n$ is equivalent to
$$
s_n+2<s_n^2
$$
which is satisfied when $s_n<-1$ or $s_n>2$. The latter is true by induction: $s_1=5>2$; suppose $s_k>2$; then $s_{k+1}=sqrt{s_k+2}>sqrt{2+2}=2$.
Your proof is good as well.
The sequence is bounded below by $0$, hence it converges. If $l$ is its limit, then $lge0$ and
$$
l=lim_{ntoinfty}s_n=lim_{ntoinfty}s_{n+1}=sqrt{l+2}
$$
Therefore $l=2$.
Actually, $lim_{ntoinfty}s_{n+1}$ should really be $lim_{ntoinfty}s'_n$, where
$$
s'_n=s_{n+1}
$$
It's essentially going through the definition of limit to show that $(s_n)$ converges if and only if $(s'_n)$ and, in this case, they have the same limit.
Since $s'_n=sqrt{s_n+1}$, the theorems on limits also allow us to say that $$l=lim_{ntoinfty}s'_n=lim_{ntoinfty}sqrt{s_n+2}=sqrt{l+2}$$
Note: I wouldn't use $s_{n-1}$, but it's just personal preference.
answered Dec 15 '18 at 16:08
egregegreg
182k1485203
182k1485203
add a comment |
add a comment |
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$begingroup$
It is the tail that matters. The tail of $s_{n-1}$ and $s_n$ are the same. So they converge to the same thing.
$endgroup$
– user9077
Dec 15 '18 at 15:55
1
$begingroup$
What happens to $n-1$ as $ntoinfty?$
$endgroup$
– saulspatz
Dec 15 '18 at 15:55