Number Systems -Sequence and Series

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sukhman: Master | Next Rank: 500 Posts; Posts: 202; Joined: Sun Sep 08, 2013 11:51 amFollowed by: 2 members; Thanked: 3 times

by sukhman » Thu Oct 17, 2013 8:22 am

36^2 + 37^2 + 38^2 + 39^2 + 40^2 + 41^2 + 42^2 + 43^2 + 44^2 =
(A) 14400 (B) 14440 (C) 14460 (D) 14500 (E) 14520

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Source: Beat The GMAT — Problem Solving | Thu Oct 17, 2013 8:22 am

theCodeToGMAT: Legendary Member; Posts: 1556; Joined: Tue Aug 14, 2012 11:18 pmFollowed by: 34 members; Thanked: 448 times; GMAT Score:650

by theCodeToGMAT » Thu Oct 17, 2013 8:48 am

Sum of series till 44 - Sum of series till 35

using formula : (N)(N+1)(2N+1)/6

(44)(45)(89)/6 - (35)(36)(71)/6

14460

R A H U L

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sanjoy18: Senior | Next Rank: 100 Posts; Posts: 81; Joined: Tue Jun 11, 2013 10:24 pmFollowed by: 1 member; Thanked: 7 times

by sanjoy18 » Thu Oct 17, 2013 12:44 pm

just calculate the last two digit of each term and then add it will give last two digit of final result which is 60 hence C

to calculate last two digit easily.
last two digit of x^2= last two digit of(50-x)^2

example: last two digit of 36^2= last two digit of 14^2 i.g. 96[/b]

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