Question

N=11 an=221 middle term is 121. find sn

Answer 1

an=a+(n-1)d

∴221=121+(6-1)d   [Here let a=121, because we don't know the exact a.

                             Now, as a=121, and it is the 6th because it is the middle

                             term and there are 11 terms. Now if you count, you will

                             get n=6 if a=121.]                        

⇒5d=100

⇒d=20

∴a=121-20×5=21  [This a is the real a]

Now, sn=n/2{2a+(n-1)d}

             =11/2{2×21+(11-1)20}

             =11/2(42+200)

             =11/2×242

             =11×121

             =1331

I hope it will help you.

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