Intereting Posts

Why can't we construct a square with area equal to a given equilateral triangle?
What is the exact motivation for the Minkowski metric?
Find the expectiation of $\displaystyle\frac{X_1 + \dots + X_m}{X_1 + \dots + X_n}$
Spotting crankery
Find $\lim\limits_{n \to \infty}\left(1-\frac{1}{2^2}\right)\left(1-\frac{1}{3^2}\right)\cdots\left(1-\frac{1}{n^2}\right)$
Can the same subset be both open and closed?
Row and column algorithm
Deduce that $f=0$ a.e.
Deriving properties of the logarithm from its integral representation
A bridge hand void in one suit
Why is $-\gamma = \int_0^1 \frac{e^{-z}-1}{z}dz+\int_1^\infty \frac{e^{-z}}{z}dz$
Does every irreducible representation of a compact group occur in tensor products of a faithful representation (and its dual)?
Does the improper integral $\int_0^\infty\sin(x)\sin(x^2)\,\mathrm dx$ converge
Integral of $1/(x^2-1)^2$
An equivalent definition of open functions

When we deal with summation notation, there are some useful computational shortcuts, e.g.:

$$\sum\limits_{i=1}^{n} (2 + 3i) = \sum\limits_{i=1}^{n} 2 + \sum\limits_{i=1}^{n} 3i = 2n + \sum\limits_{i=1}^{n}3i$$

However, I don’t think I know all the useful shortcuts here. Are there other computational tricks one should be aware of? What’s a good way for thinking about this?

More importantly, consider product notation:

$$\prod\limits_{i=1}^{n} (\sqrt{2} – \sqrt[n]{2})$$

- If $x+\frac1x=5$ find $x^5+\frac1{x^5}$.
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- How can I prove $\sqrt{2}^{\sqrt{2}^{\sqrt{2}^{\sqrt{2}^{\sqrt{2}…}}}}=2$
- Please guide me to draw this : graph of $x \geq 5$ and $y \geq 5$
- If $f$ is a strictly increasing function with $f(f(x))=x^2+2$, then $f(3)=?$

I don’t know what the shortcuts here are. What are some of the more effective ways of attacking such computations?

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Some sum identities:

$$\sum_{n=s}^t C\cdot f(n) = C\cdot \sum_{n=s}^t f(n)$$

$$\sum_{n=s}^t f(n) + \sum_{n=s}^{t} g(n) = \sum_{n=s}^t \left[f(n) + g(n)\right]$$

$$\sum_{n=s}^t f(n) – \sum_{n=s}^{t} g(n) = \sum_{n=s}^t \left[f(n) – g(n)\right]$$

$$\sum_{n=s}^t f(n) = \sum_{n=s+p}^{t+p} f(n-p)$$

$$\sum_{n=s}^j f(n) + \sum_{n=j+1}^t f(n) = \sum_{n=s}^t f(n)$$

$$\sum_{n\in A} f(n) = \sum_{n\in \sigma(A)} f(n)$$

$$\sum_{i=k_0}^{k_1}\sum_{j=l_0}^{l_1} a_{i,j} = \sum_{j=l_0}^{l_1}\sum_{i=k_0}^{k_1} a_{i,j}$$

$$\sum_{n=0}^t f(2n) + \sum_{n=0}^t f(2n+1) = \sum_{n=0}^{2t+1} f(n)$$

$$\sum_{n=0}^t \sum_{i=0}^{z-1} f(z\cdot n+i) = \sum_{n=0}^{z\cdot t+z-1} f(n)$$

$$\sum_{n=s}^t \ln f(n) = \ln \prod_{n=s}^t f(n)$$

$$c^{\left[\sum_{n=s}^t f(n) \right]} = \prod_{n=s}^t c^{f(n)}$$

$$\sum_{i=m}^n 1 = n+1-m$$

$$\sum_{i=1}^n \frac{1}{i} = H_n$$

$$\sum_{i=1}^n \frac{1}{i^k} = H^k_n$$

$$\sum_{i=m}^n i = \frac{n(n+1)}{2} – \frac{m(m-1)}{2} = \frac{(n+1-m)(n+m)}{2}$$

$$\sum_{i=0}^n i = \sum_{i=1}^n i = \frac{n(n+1)}{2}$$

$$\sum_{i=0}^n i^2 = \frac{n(n+1)(2n+1)}{6} = \frac{n^3}{3} + \frac{n^2}{2} + \frac{n}{6}$$

$$\sum_{i=0}^n i^3 = \left(\frac{n(n+1)}{2}\right)^2 = \frac{n^4}{4} + \frac{n^3}{2} + \frac{n^2}{4} = \left[\sum_{i=1}^n i\right]^2$$

$$\sum_{i=0}^n i^4 = \frac{n(n+1)(2n+1)(3n^2+3n-1)}{30} = \frac{n^5}{5} + \frac{n^4}{2} + \frac{n^3}{3} – \frac{n}{30}$$

$$\sum_{i=0}^n i^p = \frac{(n+1)^{p+1}}{p+1} + \sum_{k=1}^p\frac{B_k}{p-k+1}{p\choose k}(n+1)^{p-k+1}$$

$$\left(\sum_{i=m}^n i\right)^2 = \sum_{i=m}^n ( i^3 – im(m-1) )$$

$$\sum_{i=m}^n i^3 = \left(\sum_{i=m}^n i\right)^2 + m(m-1)\sum_{i=m}^n i$$

$$\sum_{i=m}^{n-1} a^i = \frac{a^m-a^n}{1-a}$$

$$\sum_{i=0}^{n-1} a^i = \frac{1-a^n}{1-a}$$

$$\sum_{i=0}^{n-1} i a^i = \frac{a-na^n+(n-1)a^{n+1}}{(1-a)^2}$$

$$\sum_{i=0}^{n-1} i 2^i = 2+(n-2)2^{n}$$

$$\sum_{i=0}^{n-1} \frac{i}{2^i} = 2-\frac{n+1}{2^{n-1}}$$

$$\sum_{i=0}^n {n \choose i} = 2^n$$

$$\sum_{i=1}^{n} i{n \choose i} = n2^{n-1}$$

$$\sum_{i=0}^{n} i!\cdot{n \choose i} = \sum_{i=0}^{n} {}_{n}P_{i} = \lfloor n!\cdot e \rfloor$$

$$\sum_{i=0}^{n-1} {i \choose k} = {n \choose k+1}$$

$$\sum_{i=0}^n {n \choose i}a^{(n-i)} b^i=(a + b)^n$$

$$\sum_{i=0}^n i\cdot i! = (n+1)! – 1$$

$$\sum_{i=1}^n {}_{i+k}P_{k+1} = \sum_{i=1}^n \prod_{j=0}^k (i+j) = \frac{(n+k+1)!}{(n-1)!(k+2)}$$

$$\sum_{i=0}^n {m+i-1 \choose i} = {m+n \choose n}$$

Summation of product of two functions.

$$ \sum_{i=1}^{n} f(x) g(x) $$ = $$ \sum_{i=1}^{n}f(x)∑g(x) –

[ f(x)∑g(x-1) + f(x-1)∑g(x-2) + f(x-2)∑g(x-3) + …. + f(2)∑g(1) + g(x)∑f(x-1) + g(x-1)∑f(x-2) + g(x-2)∑f(x-3) + …. + g(2)∑f(1)]$$

This is not the simplest form yet. Someone please help me to simplify and retype.

Here are the formula for the sum of the first $n$ natural numbers and the first $n$ squares. There are similar formula for the sum of the first $n$ cubes etc…

$$ \sum_{i=1}^{n} i = \frac{n(n+1)}{2}$$

$$ \sum_{i=1}^{n} i^{2} = \frac{n(n+1)(2n+1)}{6} $$

The formula for the sum of an arithmetic series is also useful:

if we know the first term $a_1$ and the last term $a_n$, and the series has $n$ terms, then the sum will be $$\frac{n(a_1+a_n)}{2}$$

I suppose $\prod\limits_{i=1}^{n}(x)$ means multiplying x n times, which is x power n. So maybe it will help, $(\sqrt{2} – \sqrt[n]{2})$ does not contain any i

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