well, ace of 1 (or the first number in the equation) =1, d=1 (or what it's added by each time), which shows that it's arithmetic), and n=1000 (the number you're trying to get)
the equation is s of n=n/2 (ace of 1 + ace of n) s of n is the sum of the numbers
so, s of n=500(1+1000)
s of n=500500
so 500,500 is the sum of the numbers from 1 to 1000.
you can find more at
http://www.YouTube.com/watch?v=VgVJrSJxkDk&feature=youtube_gdata
http://www.youtube.com/watch?v=U_8GRLJplZg&feature=youtube_gdata
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1001
1,000,000 * * * * * The 1st and 500th sum to (2*1-1)+(2*500-1) = 2*501 - 2 = 1000 The 2nd and 499th sum to (2*2-1)+(2*499-1) = 2*501 - 2 = 1000 There are 250 such sums So sum of all 500 odd numbers = 250*1000 = 250,000
The sum of all the odd numbers between 1 and 12000 is 36000000.
The sum of all the odd numbers from 1 through 100 is 10,000
501