Question
Fri November 11, 2011 By:

Find the sum of first n terms if an AP whose nth term is 5n-1. Hence find the sum of first 20 terms

Expert Reply
Fri November 11, 2011
nth term = 5n - 1
 
Put n=1, we get
 
1st term = 5-1 = 4
 
Hence sum of n terms = n(First term + nth term)/2
 
= n(4+5n - 1)/2
 
= n(5n +3)/2
 
Sum of 20 terms becomes, put n = 20
 
S20 = 20(5×20 +3)/2
 
=10(103) = 1030
 
Thank you
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