A basic and conceptual question from arithmetic progression chapter in which we need to find the sum of given arithmetic series
34 + 32 + 30 +……+10.
RS Aggarwal, Class 10, chapter 5C, question no 2(ii)
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a=34,
d=32−34=−2
an=10
or, a+(n−1)d=10
or, 34+(n−1)(−2)=10
or, −2(n−1)=−24
or, n−1=12
⇒n=13
∴S13=13/2[2(34)+12(−2)]
=13/2×2[34−12]
=13×22=286.