Question 643622
<pre>

You don't need a formula.

Suppose the sum = N, then

 N =     1 +     2 +     3 +     4 + ··· +  9996 +  9997 +  9998 +  9999

N also equals that same sum with the numbers added in the reverse order:

 N =  9999 +  9998 +  9997 +  9996 + ··· +     4 +     3 +     2 +     1


Now let's write those two equations one under the other, and add equals 
to equals term by term:

 N =     1 +     2 +     3 +     4 + ··· +  9996 +  9997 +  9998 +  9999
 N =  9999 +  9998 +  9997 +  9996 + ··· +     4 +     3 +     2 +     1
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2N = 10000 + 10000 + 10000 + 10000 + ··· + 10000 + 10000 + 10000 + 10000

Since we know there are 9999 terms on the right, that sum on the right is 
9999 times 10000 or 99990000, so that equation bercomes

2N = 99990000

Dividing both sides by 2

 N = 49995000 

Edwin</pre>