Question 643622
First do a shorter sequence to get the
right method. First find the sum of 
0 to 10
{{{ 0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55 }}}
( I did it on my calculator )
This is {{{ (10/2)*( 10 + 1 ) }}}
How about
{{{ 0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 = 91 }}}
This is {{{ (13/2)*( 13 + 1 ) }}}
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So, the formula is:
The sum of the numbers from 1 to n =
{{{ (n/2)*( n + 1 ) }}}
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{{{ n = 9999 }}}
{{{ ( 9999/2 )*( 9999 + 1 ) = 4999.5*10000 }}}
{{{ 4999.5*10000 = 49995000 }}}