Compute the sum 1 + 4 + 7 + ... + 2005 + 2008.
From an online summation calculator:
n/2 ( 2a + (n-1)d) n = 670 a = 1 d = 3
670/2 (2+ 669(3)) = 673015
The formula for the sum of an AP is
S=n/2(2a+L)
+37159 
