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=Sn−Sn−1 =3n2+5n−3(n−1)2−5(n−1)[∵Sn=3n2+5n(given)] =3n2+5n−3n2−3+6n−5n+5 an=6n+2⋯(i) Orak=6k+2=164[∵ak=164(given)] ⇒6k=164−2=162 ∴k=27