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Sn=3n×3n_4n find an

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Sn=3n×3n_4n find an

Posted by Dk Singh 6 years, 4 months ago

CBSE > Class 10 > Mathematics

1 answers

Ram Kushwah 6 years, 4 months ago

Sn=3n×3n - 4n=9n^2-4n---------------- (1)

putting n-1 in place of n

Sn-1=9(n-1)(n-1)-4(n-1)

=9n^2-18n+9-4n+1

Sn-1=9n^2-22n+10-------------------------(2)

We know that

Sum of n terms = sum of n-1 terms + an

Sn=Sn-1 + an

an=Sn-Sn-1

Putting value of Sn and Sn-1

an=9n^2-4n - (9n^2-22n+10)

=9n^2-4n-9n^2+22n-10

=18n-10

Hence an=18n-10

0Thank You

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CBSE > Class 10 > Mathematics