Arithmetic Progression question on finding the 25th term

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In AP, sum of n terms is $\dfrac{3n^2 + 5n}{2}$. Find 25th term.

My work :

$S_n = \dfrac{n}{2}\left({3n + 5}\right)$

$2a + (n-1)d = \left({3n + 5}\right)$

$2a + 24d = 80$

$a + 12d = 40$

13th term is 40.

Bit we need 25th term! Please help

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1 Answer

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Hint: The $n^{th}$ term is the difference of sum of the first $n$ terms and sum of first $(n-1)$ terms. Mathematically, $$T_n=S_n-S_{n-1}$$

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