Question

If nth term of an A.P is 10n-6 then find sum of first n terms. (2 marks question)​

Answer 1

nth term = 10n - 6

by the formula,

$a_{1} = 10(1) - 6$

    $= 10 - 6$

    $= 4$

$a_{2} =10 (2) - 6$

    $= 20 - 6$

    $= 14$

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

  $= 14 - 4$

  $= 10$

Now,

$S_{n} = \frac{n}{2} [2a + (n-1) d ]$

    $= \frac{n}{2} [2 (4) + (n-1) 10]$

   $= \frac{n}{2} [ 8 + 10n - 10]$

   $= n [ 4 + 10n - 10 ]$

   $= 4n + 10n^{2} - 10n$

  $= 10n^{2} - 6n$

Hope it helps u