The $n t h$ term of AP is given by $T n = 5 n - 3$ . Find sum of first $20$ term of $A . P$ .

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Solution

$T_{n} = 5 n - 3$
$T_{1} = 5 (1) - 3 = 2$
$T_{2} = 5 (2) - 3 = 7$
$d = T_{2} - T_{1}$
$= 7 - 2 = 5$
$a = T_{1} = 2$
$S_{20} = \frac{20}{2} [2 a + 19 d]$
$= 10 (4 + 95)$
$= 990$ .

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