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Question

Question 25
In an AP, if Sn=3n2+5n and ak=164 , then find the value of k.

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Solution

nth term of an AP,
an=SnSn1
=3n2+5n3(n1)25(n1) [Sn=3n2+5n(given)]
=3n2+5n3n23+6n5n+5
an=6n+2 (i)
Or ak=6k+2=164 [ak=164(given)]
6k=1642=162
k=27

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