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Compute the sum 1 + 4 + 7 + ... + 2005 + 2008.

 Aug 17, 2020
 #1
avatar+26856 
+1

From an online summation calculator:

 

 

n/2 ( 2a + (n-1)d)            n = 670    a = 1   d = 3

670/2 (2+ 669(3)) = 673015

 Aug 17, 2020
edited by ElectricPavlov  Aug 17, 2020
 #2
avatar+110762 
+2

The formula for the sum of an AP is

 

S=n/2(2a+L)

 Aug 17, 2020

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