Arithmetic Series

**juliak** · November 17th 2008, 01:01 AM

1. Find the equation that gives the sum of the first n positive integer
^
the answer given was n(n+1)/2 - but I don't know how to get it

2. Show that the sum of the first n odd integers is equal to the perfect square n^2

3. Show that the sum of the first n even integers is equal to n^2 + n

**clic-clac** · November 17th 2008, 01:52 AM

Let $n$ be a positive integer. We will write $S_{n}$ the sum of the first $n$ integers.

1)
$1 + 2 + ... + n = S_{n}$
$n + (n-1) + ... + 1 = S_{n}$

So if we sum these two equations, we get

$(n+1) + ((n-1)+2) + ... + (n+1) = 2S_{n}$ , that is to say

$(n+1) + (n+1) + ... + (n+1) = n(n+1) = 2S_{n}$

Therefore $S_{n}=\frac{n(n+1)}{2}$

2)
What are the first $n$ odd integers? $1,3,...,2n-1$
So their sum is
$\sum\limits_{k=1}^{n}(2k-1)=\sum\limits_{k=1}^{n}2k-\sum\limits_{k=1}^{n}1=2\sum\limits_{k=1}^{n}k-n=n(n+1)-n=n(n+1-n)=n^{2}$

3)
It's easier than 2)

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