All Questions
Tagged with summation algebra-precalculus
977
questions
-1
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0
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54
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Summation involving the closest integer to $\sqrt n$ [closed]
Let $f(n)$ be the integer closest to $\sqrt n$. Evaluate
$$\sum_{n=1}^\infty\frac{\left(\frac32\right)^{f(n)}+\left(\frac32\right)^{-f(n)}}{\left(\frac32\right)^n}$$
In this question, I was able to ...
- 9
2
votes
0
answers
102
views
$x_{1}=\frac{1}{x_{1}}+x_{2}=\frac{1}{x_{2}}+x_{3}=\ldots=\frac{1}{x_{n-1}}+x_{n}=\frac{1}{x_{n}}$; prove that $x_{1}=2\cos\frac{\pi}{n+2}$ [closed]
If $x_{1}, x_{2}, x_{3}, \ldots, x_{n}$ are $n\geq 2$ positive real numbers such that $
x_{1}=\frac{1}{x_{1}}+x_{2}=\frac{1}{x_{2}}+x_{3}=\ldots=\frac{1}{x_{n-1}}+x_{n}=\frac{1}{x_{n}}$, prove that $...
- 1,122
2
votes
1
answer
82
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If $M:=\sum\limits_{k=1}^{\frac{n(n+1)}{2}}\lfloor\sqrt{2k}\rfloor$ How to Find $\frac{n^3+2n}{M}$?
$$M:=\sum\limits_{k=1}^{\frac{n(n+1)}{2}}\lfloor\sqrt{2k}\rfloor$$
Find $\frac{n^3+2n}{M}$
This problem was on a problem book.
It is easy to find $M$
If $n$ is odd, $\ m=\frac{n+1}{2} $ and $$M= \...
- 3,338
18
votes
12
answers
17k
views
How to prove this binomial identity $\sum_{r=0}^n {r {n \choose r}} = n2^{n-1}$?
I am trying to prove this binomial identity $\displaystyle\sum_{r=0}^n {r {n \choose r}} = n2^{n-1}$ but am not able to think something except induction,which is of-course not necessary (I think) here,...
- 2,973
1
vote
0
answers
48
views
Is there a rule for using parentheses or brackets after the summation symbol to indicate what is included in the sum? [duplicate]
Using parentheses or brackets removes ambiguity but is it necessary?
- 19
1
vote
3
answers
93
views
Evaluating $\sum_{\substack{i+j+k=n \\ 0\leq i,j,k\leq n}} 1$
I need to find the sum $$\sum_{\substack{i+j+k=n \\ 0\leq i,j,k\leq n}} 1$$
For $n=1$ we have the admissible values of $(i,j,k)$ as: $(1,0,0),(0,1,0), (0,0,1)$ $$\sum_{\substack{i+j+k=1 \\ 0\leq i,j,...
- 397k
2
votes
1
answer
104
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Enquiry on a claim in Titchmarsh. [closed]
There is a claim on p.107 of Titchmarsh's ''The theory of the Riemann zeta function'', that if $0<a<b \leq 2a$ and $t>0$, then
the bound
$$\sum_{a <n <b} n^{-\frac{1}{2}-it} \ll (a/t)^{...
- 92.6k
1
vote
2
answers
187
views
summing binomial coefficiens related
$$
\mbox{If}\quad
s_{n} = \sum_{k = 0}^{n}\left(-4\right)^{k}
\binom{n + k}{2k},\quad\mbox{how to prove}\quad s_{n + 1} + 2s_{n} + s_{n - 1} = 0\ ?.
$$
One of my student had this question in his exam....
- 47.4k
2
votes
2
answers
398
views
Find the value of $S_1+S_2$
Knowing that $$\sum_{k=1}^{\infty}\frac{1}{k^2}=\frac{\pi^2}{6}$$
and $$S_i=\sum_{k=1}^{\infty}\frac{i}{(36k^2-1)^i}$$
Find value of $S_1+S_2$
i tried splitting:
$$\frac{1}{36k^2-1}=\frac{1}{2}\...
- 11
-1
votes
1
answer
55
views
Expressing $\sum_{b=0}^a\sum_{c=0}^b c$ in terms of $a$ [closed]
Summation with the form:
$$\sum_{b=0}^a\sum_{c=0}^b c$$
I am not aware of any rule about chaining sums and getting a value in terms of the variable $a$.
- 433
0
votes
1
answer
15
views
Solve for variable f when f is in a denominator function of a sum
I have the following equation which I need to solve for f:
$\frac{X}{fY} = \sum_t^{T-1}\frac{A(t)}{f\cdot B(t)+1}$
While this seems very solvable, it has stumped an entire group of physics students. ...
- 125
8
votes
3
answers
565
views
Finding sum : ${\mathop{\sum\sum\sum\sum}_{0\le i\lt j\lt k\lt l\le n }} \,1$
Finding the value of: $${\mathop{\sum\sum\sum\sum}_{0\le i\lt j\lt k\lt l\le n }} 1$$
I know a similar question was asked on this site earlier, but I couldn't understand the method used there.
Link ...
- 3,845
3
votes
2
answers
117
views
$1\binom{20}1+2\binom{20}2+3\binom{20}3+\dots+19\binom{20}{19}+20\binom{20}{20}$
$$1\binom{20}1+2\binom{20}2+3\binom{20}3+\dots+19\binom{20}{19}+20\binom{20}{20}$$
I solved it by letting the sum be $S$, then adding the sum to itself but taking the terms from last to first and then ...
- 86
0
votes
1
answer
54
views
Why is $\sum_{m=0}^{\lfloor xs\rfloor} 2 \binom{s}{m} p^m (1-p)^{s-m} \leq 2\exp{\left(-\frac{2(\lfloor xs\rfloor - sp)^2}{s}\right)}$
I am trying to understand few of the mathematical steps I have encountered in a paper, there are two of them
(a) $\sum_{m=0}^{\lfloor xs\rfloor} 2 \binom{s}{m} p^m (1-p)^{s-m} \leq 2\exp{\left(-\frac{...
- 141
1
vote
1
answer
80
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Sum related to Binomial Coefficients [duplicate]
Calculate:- $$\sum_{r=1}^{2023} \frac{(-1)^{r-1}r}{2024 \choose r}$$
And generalise the result if possible.
I've tried to reduce this to a telescopic sum but could not do it.
I've also made a ...
- 19
3
votes
4
answers
84
views
Finding and proofing a closed formula for $\sum_{n=1}^k\sqrt{1+\frac{1}{n^2}+\frac{1}{(n+1)^2}}$
I want to find and proof a closed formula for the following sum $$\sum_{n=1}^k\sqrt{1+\frac{1}{n^2}+\frac{1}{(n+1)^2}}=\sqrt{1+\frac{1}{1^2}+\frac{1}{2^2}}+\sqrt{1+\frac{1}{2^2}+\frac{1}{3^2}}+\dots +\...
- 276k
0
votes
1
answer
54
views
On a strange step in the proof regarding a maximal problem.
As far as I'm concerned to show that something is true, proving that something is true for an example is never enough, you have to be able to prove that it is true for all statements/numbers with the ...
- 289
0
votes
0
answers
39
views
Simplification of the ratio between series
I have been trying to solve a problem i posed to myself in the applied sciences, and technically, i did (though it is not of any practical use). But the problem is that the solution is, well, not ...
- 1
1
vote
1
answer
122
views
$(an)_{n\in\mathbb{N}^*}$ is a sequence, so that $a_1=1$, for all $n\geq 2$, $a_n=a_1\cdot...\cdot a_{n-1}+1$. $\sum_{n=1}^{m}\frac{1}{a_n}<M$, find M
This is a problem I came across on another exam. It is as follows:
Let $(an)_{n\in\mathbb{N}^*}$ a sequence such that $a_1=1$ and for all $n\geq 2$, $a_n=a_1\cdot...\cdot a_{n-1}+1$. Find the real ...
- 117k
-3
votes
3
answers
106
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Why is $\sum_{n=1}^{k}\frac{1}{n^2+n}=\frac{k}{k+1}$ [duplicate]
$$\sum_{n=1}^{k}\frac{1}{n^2+n}=\frac{k}{k+1}$$
I don't think that this summation requires too much context as this is a Q&A site, but I was just wondering why the summation is evaluated so nicely....
- 1,254
0
votes
3
answers
88
views
Evaluate using combinatorial argument or otherwise :$\sum_{i=0}^{n-1}\sum_{j=i+1}^{n}\left(j\binom{n}{i}+i\binom{n}{j}\right)$
Evaluate using combinatorial argument or otherwise $$\sum_{i=0}^{n-1}\sum_{j=i+1}^{n}\left(j\binom{n}{i}+i\binom{n}{j}\right)$$
My Attempt
By plugging in values of $i=0,1,2,3$ I could observe that ...
- 47.4k
0
votes
0
answers
45
views
Inequality with Products and Sums
I need help to find a proof for the following inquality.
Assuming that $ 0 \leq c_i \leq 1 $ and $ 0 \leq d_i \leq 1 $, show that
$$
\prod_{i=1}^N (c_i + d_i - c_i d_i) \geq \prod_{i=1}^N c_i + \prod_{...
- 778
3
votes
4
answers
163
views
Proving $\sum_{i=0}^n (-1)^i\binom{n}{i}\binom{m+i}{m}=(-1)^n\binom{m}{m-n}$
I am trying to prove the following binomial identity:
$$\sum_{i=0}^n (-1)^i\binom{n}{i}\binom{m+i}{m}=(-1)^n\binom{m}{m-n}$$
My idea was to use the identity
$$\binom{m}{m-n}=\binom{m}{n}=\sum_{i=0}^n(-...
- 47.4k
1
vote
3
answers
74
views
Attempt at creating a formula relating debt, payments and interest
I tried writing down a formula relating a given debt and interest to the periodic payments and number of payments.
So let's say someone starts off with a debt of $D$. The periodic interest is $r$ (for ...
- 42.8k
4
votes
2
answers
392
views
Find the sum of series: $\frac{1}{\sqrt{1}+\sqrt{2}}+\frac{1}{\sqrt{3}+\sqrt{4}}+...+\frac{1}{\sqrt{97}+\sqrt{98}}+\frac{1}{\sqrt{99}+\sqrt{100}}$
Find the sum of series:
$$\frac{1}{\sqrt{1}+\sqrt{2}}+\frac{1}{\sqrt{3}+\sqrt{4}}+\frac{1}{\sqrt{5}+\sqrt{6}}+...+\frac{1}{\sqrt{97}+\sqrt{98}}+\frac{1}{\sqrt{99}+\sqrt{100}}$$
My Attempt:
I tried ...
- 1,512
2
votes
6
answers
171
views
Why does $ 1+2+3+\cdots+p = {(1⁄2)}\cdots(p+1) $ [duplicate]
I saw this from Project Euler, problem #1:
If we now also note that $ 1+2+3+\cdots+p = {(1/2)} \cdot p\cdot(p+1) $
What is the intuitive explanation for this? How would I go about deriving the ...
- 565
10
votes
2
answers
417
views
If $z\in\mathbb C$ with $|z|\leqslant\frac{4}{5}$, then $\sum_{n\in S}z^n\neq-\frac{20}{9}$
Let $z$ be a complex number with $|z|\le\tfrac{4}{5}$. If $S\subset\mathbb N^+$ is a finite set, then I'd like to show that
$$\sum_{n\in S}z^n\neq-\frac{20}{9}\,.$$
This problem is from an exam in ...
- 81
3
votes
0
answers
35
views
Prove that $\sum_{r=1}^n (-1)^{r-1}(1+\frac{1}{2}+\frac{1}{3}+...\frac{1}{r}) \binom{n}{r} =\frac{1}{n}$. [duplicate]
Prove that $\sum_{r=1}^n (-1)^{r-1}(1+\frac{1}{2}+\frac{1}{3}+...\frac{1}{r}) \binom{n}{r} =\frac{1}{n}$. Where $\binom{n}{r}$ represents 'n choose r'.
I tried to simplify this expression by first ...
- 334
1
vote
1
answer
41
views
Conditions for $ \sum_{x\in G} f(x) = \sum_{x\in f(G)} x $ to hold
i have a question
what are the condition on the function $f$ ?
so that this equality hold :
$ \sum_{x\in G} f(x) = \sum_{x\in f(G)} x $
is $f$ surjective is a necessary for this question ?
please ...
- 2,407
1
vote
2
answers
82
views
Upper rectangle area sum to approximate 1/x between $1\leq x\leq 3$
I am trying to figure out how to use rectangles to approximate the area under the curve $1/x$ on the interval $[1,3]$ using $n$ rectangle that covers the region under the curve as such.
Here is what I ...
- 2,747