Question

If Sn ,the sum of first n terms of an AP is given by Sn=3n²-4n ,then find the sum of 20 terms.
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Answer 1

Solution

⟹Given Sum of n terms is Sn =3n²-4n

⟹So Sum of n−1 terms is Sn−1 3(n − 1)² −

⟹=3n² − 6n + 3 − 4n + 4

⟹3n² − 10n + 7

• Sum of n terms is equal to sum of n −1 terms plus n th term

⟹n th term = Sn − Sn − 1 =3n² − 4n − (=3n² − 10n + 7)

n th term = 6n − 7