Question

Find an=2to the power n find s3
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Answer 1

Answer:

Given, Sn=2n2+n

Now, a1=S1=2(1)2+1=3

a2=S2−S1=2(2)2+2−3=7

a3=S3−S2=[2(3)2+3]−[2(2)2+2]=21−10=11

a4=S3−S3=[2(4)2+4]−[2(3)2+3]=36−21=15 and so on.

Therefore the required AP is 3,7,11,15

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