Question

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

Answer 1

`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]