Wednesday, July 3, 2013

find the sum of the integer between 30 and 1000 that are divisible by 3

they are 33, 36, 39,...,999 (an arithmetic progression) 
so... 
a%5B1%5D=33 

a%5Bn%5D=999 

n=%28a%5Bn%5D-a%5B1%5D%29%2Fd%2B1+=+%28999-33%29%2F3%2B1=323 
As for the sum... 

s%5Bn%5D=%28n%2F2%29%28a%5B1%5D%2Ba%5Bn%5D%29=%28323%2F2%29%28999%2B33%29=166668 


:)
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