Answers

2016-02-06T13:33:44+05:30
Given series is 7,12,17..........
a=7, d=5, sum =2542,
sum = (a+an/2)n
2542 = [7+(a+(n-1)d/2]n
2542 = [7+(7+(n-1)5)/2]n
2542*2 =(5n+9)n
5n^2+9n=5084
n=31//
a31=a+(n-1)d
a31=7+(31-1)5
=7+30*5
=7+150
ie, a31=157.
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