the sum of first n consecutive positive

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4meonly: Legendary Member; Posts: 891; Joined: Sat Aug 16, 2008 4:21 amFollowed by: 1 member; Thanked: 27 times; GMAT Score:660(

by 4meonly » Sun Aug 24, 2008 10:48 pm

the sum of first n consecutive positive odd integers is n^2. What is the sum of odd integers from 25 to 79, inclusive?
A. 1252
b. 1377
c. 1431
d. 1456
e. 2916

OA is D

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Source: Beat The GMAT — Problem Solving | Sun Aug 24, 2008 10:48 pm

carefreeamit: Junior | Next Rank: 30 Posts; Posts: 14; Joined: Fri Aug 22, 2008 9:51 pm; Thanked: 3 times

by carefreeamit » Sun Aug 24, 2008 11:10 pm

25 is 13th odd number and 79 is 40th odd number.
sum of odd integers from 25 to 79, inclusive = sum of odd integers till 79 - sum of odd integers till 23 (remember 25 needs to be included in the total)

= 40^2 - 12^2
= 1600 - 144
= 1456

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