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Prove that a sequence in an A.P., if the...

Prove that a sequence in an A.P., if the sum of its `n` terms is of the form `A n^2+B n ,w h e r eA ,B` are constants.

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Let, `a` be the first term of the A.P.
and `d` be a common difference.
Then sum of the `n` terms of the A.P= `n/2[2a+(n-1)d]`
`=>(2an)/2+(n(n-1)d)/2`
...
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