Question

The sum of n terms of an AP is given by Sn = 2n2 - 3n. find its 21st term.

Answer 1

$S _{n} = {2n}^{2} + 3n$

$S_{1} = 5 = a_{1}$

$S_{2} = a_{1} \: \: + a_{2} = 14$

$a_{2} = 9$

$d = a_{2} - a_{1} = 4$

$a_{16} = a_{1} + 15d$

$= 5 + 15(4)$

$= 65$

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the 16 term of the AP is 65

hope its help u