Find the 21st partial sum of the sequence: -31, -19, -7, 5...?

1 Answer
Binayaka C.
May 14, 2018

#21# st partial sum is #1869#

Explanation:

This is arithmetic progression of which first term is #a=-31#

common difference is #d= -19-(-31)=12 ; n =21#

#S_21=n/2{2a+(n-1)d}=21/2{2*(-31)+(21-1)*12# or

#S_21=21/2(-62+20*12)= 21/2 (-62+240)=21/2*178# or

#S_21=1869 :. 21# st partial sum is #1869# [Ans]

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