Question

Fiind the sum of last ten terms of the A.P.: 8,10,12,14,..,126.

Answer 1

According to the question,
we invert this series ,
126 , 124 , 122 ,........8

now,
first term of new AP is = 126
common difference = -2

use formula,
$S_n$ =n/2 {2a +(n-1)d

$S_{1}0$=10/2{2 × 126 +(10-1)(-2)}
=5{252 -18}
=5 × 234
=1170