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Help:Tough problem on exponential sequence

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by aloneontheedge » Sat Jul 24, 2010 10:56 am
praveen_gmat wrote:What is the sixtieth term in the following sequence? 1, 2, 4, 7, 11, 16, 22...
(A) 1,671
(B) 1,760
(C) 1,761
(D) 1,771
(E) 1,821
Every term increases in the following pattern
1 2 4 7 11 16
First diff = 1
2nd diff = 2
3rd diff = 3 and so on
to find the sixtieth term,just find the sum of the numbers increasing by differences + 1
for example,In the sequence provided 5th tem is 11
to get this we take sum of first four natural numbers which is 10 + 1 =11
similiarly to find 60th term take sum of 59 natural numbers + 1 =1771
Pick D
Last edited by aloneontheedge on Sat Jul 24, 2010 12:15 pm, edited 1 time in total.
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by akdayal » Sat Jul 24, 2010 10:58 am
My Ans : D 1771
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by praveen_gmat » Sat Jul 24, 2010 11:08 am
@ aloneontheedge...

I understood your solution .. but guess you made a calculation mistake .. 1771 is the answer.

Thanks for the help !
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by sreak1089 » Sat Jul 24, 2010 11:24 am
Pattern is: 1 2 4 7 11 16 22 29
Differences are: 1 2 3 4 5 .....

Thus each term in the sequence can be defined as tn = Sigma(n-1) + 1
Thus t60 = (59*60)/2 + 1 = 1770 + 1 = 1771.

D is the answer.
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by aloneontheedge » Sat Jul 24, 2010 12:14 pm
praveen_gmat wrote:@ aloneontheedge...

I understood your solution .. but guess you made a calculation mistake .. 1771 is the answer.

Thanks for the help !
So stupid i am, will correct it. thanks for pointing it out
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