How to calculate $lim_{n to infty} sum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}$?
$begingroup$
How to calculate $displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}$?
My try:
begin{align}
lim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}
&=displaystyle lim_{n to infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}}
\&=displaystylelim_{n to infty}int_{0}^{1}frac{dx}{sqrt{1+frac{1}{n}-x^2}}
\&=displaystylelim_{n to infty}arctan sqrt{n}
\&=frac{pi}{2}
end{align}
But,
I'm not sure whether this's right because I'm not sure whether the second equality is right.
Any helps and new ideas will be highly appreciated!
real-analysis calculus limits
$endgroup$
add a comment |
$begingroup$
How to calculate $displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}$?
My try:
begin{align}
lim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}
&=displaystyle lim_{n to infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}}
\&=displaystylelim_{n to infty}int_{0}^{1}frac{dx}{sqrt{1+frac{1}{n}-x^2}}
\&=displaystylelim_{n to infty}arctan sqrt{n}
\&=frac{pi}{2}
end{align}
But,
I'm not sure whether this's right because I'm not sure whether the second equality is right.
Any helps and new ideas will be highly appreciated!
real-analysis calculus limits
$endgroup$
$begingroup$
As written is not correct because in the limit $k/n$ changed to $x,$ however, I believe the final answer is correct.
$endgroup$
– Will M.
Dec 21 '18 at 5:39
add a comment |
$begingroup$
How to calculate $displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}$?
My try:
begin{align}
lim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}
&=displaystyle lim_{n to infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}}
\&=displaystylelim_{n to infty}int_{0}^{1}frac{dx}{sqrt{1+frac{1}{n}-x^2}}
\&=displaystylelim_{n to infty}arctan sqrt{n}
\&=frac{pi}{2}
end{align}
But,
I'm not sure whether this's right because I'm not sure whether the second equality is right.
Any helps and new ideas will be highly appreciated!
real-analysis calculus limits
$endgroup$
How to calculate $displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}$?
My try:
begin{align}
lim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}
&=displaystyle lim_{n to infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}}
\&=displaystylelim_{n to infty}int_{0}^{1}frac{dx}{sqrt{1+frac{1}{n}-x^2}}
\&=displaystylelim_{n to infty}arctan sqrt{n}
\&=frac{pi}{2}
end{align}
But,
I'm not sure whether this's right because I'm not sure whether the second equality is right.
Any helps and new ideas will be highly appreciated!
real-analysis calculus limits
real-analysis calculus limits
edited Dec 21 '18 at 9:13
Lorenzo B.
1,8602520
1,8602520
asked Dec 21 '18 at 5:21
ZeroZero
37510
37510
$begingroup$
As written is not correct because in the limit $k/n$ changed to $x,$ however, I believe the final answer is correct.
$endgroup$
– Will M.
Dec 21 '18 at 5:39
add a comment |
$begingroup$
As written is not correct because in the limit $k/n$ changed to $x,$ however, I believe the final answer is correct.
$endgroup$
– Will M.
Dec 21 '18 at 5:39
$begingroup$
As written is not correct because in the limit $k/n$ changed to $x,$ however, I believe the final answer is correct.
$endgroup$
– Will M.
Dec 21 '18 at 5:39
$begingroup$
As written is not correct because in the limit $k/n$ changed to $x,$ however, I believe the final answer is correct.
$endgroup$
– Will M.
Dec 21 '18 at 5:39
add a comment |
2 Answers
2
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oldest
votes
$begingroup$
$$frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} preceq frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}} leq frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}} $$
(the symbole $preceq$ means: It is lower than form a $nin mathbb{N}$ to later)
But $$lim_{nto infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}}=int_0^1arcsin(x)dx=frac{pi}{2} $$ and $$lim_{nto infty} frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} =arcsin left(frac{1}{sqrt {1+epsilon}}right)$$ and
$$lim_{epsilon to 0^+} arcsinleft(frac{1}{sqrt {1+epsilon}}right)=frac{pi}{2}.$$
$endgroup$
add a comment |
$begingroup$
To use integral method rigorously, I came up with a new solution.
Notice that(due to the monotonicity)
$$ displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}leint_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}+frac{1}{sqrt{n}}$$
Then we have
$$displaystylelim_{ntoinfty}displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylelim_{ntoinfty}displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}ledisplaystylelim_{ntoinfty}int_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}$$
Considering
$$displaystyleintfrac{dx}{sqrt {n^2+n-x^2}}=arctanfrac{x}{sqrt {n^2+n-x^2}}$$
Then we can arrive at
$$displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}=frac{pi}{2}$$
$endgroup$
add a comment |
Your Answer
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2 Answers
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2 Answers
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active
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$begingroup$
$$frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} preceq frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}} leq frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}} $$
(the symbole $preceq$ means: It is lower than form a $nin mathbb{N}$ to later)
But $$lim_{nto infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}}=int_0^1arcsin(x)dx=frac{pi}{2} $$ and $$lim_{nto infty} frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} =arcsin left(frac{1}{sqrt {1+epsilon}}right)$$ and
$$lim_{epsilon to 0^+} arcsinleft(frac{1}{sqrt {1+epsilon}}right)=frac{pi}{2}.$$
$endgroup$
add a comment |
$begingroup$
$$frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} preceq frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}} leq frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}} $$
(the symbole $preceq$ means: It is lower than form a $nin mathbb{N}$ to later)
But $$lim_{nto infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}}=int_0^1arcsin(x)dx=frac{pi}{2} $$ and $$lim_{nto infty} frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} =arcsin left(frac{1}{sqrt {1+epsilon}}right)$$ and
$$lim_{epsilon to 0^+} arcsinleft(frac{1}{sqrt {1+epsilon}}right)=frac{pi}{2}.$$
$endgroup$
add a comment |
$begingroup$
$$frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} preceq frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}} leq frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}} $$
(the symbole $preceq$ means: It is lower than form a $nin mathbb{N}$ to later)
But $$lim_{nto infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}}=int_0^1arcsin(x)dx=frac{pi}{2} $$ and $$lim_{nto infty} frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} =arcsin left(frac{1}{sqrt {1+epsilon}}right)$$ and
$$lim_{epsilon to 0^+} arcsinleft(frac{1}{sqrt {1+epsilon}}right)=frac{pi}{2}.$$
$endgroup$
$$frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} preceq frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+frac{1}{n}-(tfrac{k}{n})^2}} leq frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}} $$
(the symbole $preceq$ means: It is lower than form a $nin mathbb{N}$ to later)
But $$lim_{nto infty} frac{1}{n}displaystylesum_{k=1}^{n}frac{1}{sqrt {1-(tfrac{k}{n})^2}}=int_0^1arcsin(x)dx=frac{pi}{2} $$ and $$lim_{nto infty} frac{1}{n}sum_{k=1}^{n}frac{1}{sqrt {1+epsilon-(tfrac{k}{n})^2}} =arcsin left(frac{1}{sqrt {1+epsilon}}right)$$ and
$$lim_{epsilon to 0^+} arcsinleft(frac{1}{sqrt {1+epsilon}}right)=frac{pi}{2}.$$
edited Dec 21 '18 at 10:25
amWhy
1
1
answered Dec 21 '18 at 9:36
DarmanDarman
538112
538112
add a comment |
add a comment |
$begingroup$
To use integral method rigorously, I came up with a new solution.
Notice that(due to the monotonicity)
$$ displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}leint_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}+frac{1}{sqrt{n}}$$
Then we have
$$displaystylelim_{ntoinfty}displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylelim_{ntoinfty}displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}ledisplaystylelim_{ntoinfty}int_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}$$
Considering
$$displaystyleintfrac{dx}{sqrt {n^2+n-x^2}}=arctanfrac{x}{sqrt {n^2+n-x^2}}$$
Then we can arrive at
$$displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}=frac{pi}{2}$$
$endgroup$
add a comment |
$begingroup$
To use integral method rigorously, I came up with a new solution.
Notice that(due to the monotonicity)
$$ displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}leint_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}+frac{1}{sqrt{n}}$$
Then we have
$$displaystylelim_{ntoinfty}displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylelim_{ntoinfty}displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}ledisplaystylelim_{ntoinfty}int_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}$$
Considering
$$displaystyleintfrac{dx}{sqrt {n^2+n-x^2}}=arctanfrac{x}{sqrt {n^2+n-x^2}}$$
Then we can arrive at
$$displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}=frac{pi}{2}$$
$endgroup$
add a comment |
$begingroup$
To use integral method rigorously, I came up with a new solution.
Notice that(due to the monotonicity)
$$ displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}leint_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}+frac{1}{sqrt{n}}$$
Then we have
$$displaystylelim_{ntoinfty}displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylelim_{ntoinfty}displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}ledisplaystylelim_{ntoinfty}int_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}$$
Considering
$$displaystyleintfrac{dx}{sqrt {n^2+n-x^2}}=arctanfrac{x}{sqrt {n^2+n-x^2}}$$
Then we can arrive at
$$displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}=frac{pi}{2}$$
$endgroup$
To use integral method rigorously, I came up with a new solution.
Notice that(due to the monotonicity)
$$ displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}leint_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}+frac{1}{sqrt{n}}$$
Then we have
$$displaystylelim_{ntoinfty}displaystyleint_{0}^{n}frac{dx}{sqrt {n^2+n-x^2}} le displaystylelim_{ntoinfty}displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}ledisplaystylelim_{ntoinfty}int_{1}^{n}frac{dx}{sqrt {n^2+n-x^2}}$$
Considering
$$displaystyleintfrac{dx}{sqrt {n^2+n-x^2}}=arctanfrac{x}{sqrt {n^2+n-x^2}}$$
Then we can arrive at
$$displaystylelim_{n to infty} displaystylesum_{k=1}^{n}frac{1}{sqrt {n^2+n-k^2}}=frac{pi}{2}$$
answered Dec 21 '18 at 9:30
ZeroZero
37510
37510
add a comment |
add a comment |
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$begingroup$
As written is not correct because in the limit $k/n$ changed to $x,$ however, I believe the final answer is correct.
$endgroup$
– Will M.
Dec 21 '18 at 5:39