In probability, what's this symbol called?

Question

anonymous · Answer

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mathteacher1729 · Answer

The $\Sigma $ is called "sigma". It is the greek letter "S" and stands for "sum". The $_nC_r$ is a shorthand of writing "n choose r" which describes quite a few things, all of which are related, all dealing with how many ways there are to arrange r objects chosen from a set of n objects. It is also a way of finding binomial coefficients. :) This is a fantastic discussion of permutations and combinations here: http://www.mathsisfun.com/combinatorics/combinations-permutations.html

anonymous · Answer

n Choose r

$$n C r = \frac{n!}{r!(n-r)!}$$

anonymous · Answer

$$\left(\begin{matrix}n \ k\end{matrix}\right)$$ this it other notation of combination

anonymous · Answer

Thanks, i know that it's a combination (the nCr) but i'm not sure about the sigma. May you please help explain that in greater detail?

anonymous · Answer

sigma is a sum symbol
$$\sum_{i=0}^{n}i=0+1+2+3+...+n$$
I think math teacher can explain more :P

anonymous · Answer

like a factorial? except you're adding?

anonymous · Answer

not really

anonymous · Answer

$$\sum_{i=0}^{n}i^2=0^2+1^2+2^2+3^2+...+n^2$$
it depends on expression, now its $i^2$ so we need to add all squares
if you are familiar with programming then it's like a loop :D

anonymous · Answer

oh well