Question

Find the sum of last ten terms of the AP : 8, 10, 12,...., 126.

Answer 1

For finding, the sum of last ten terms, we write the given AP in reverse order i.e, 126, 124, 122,..., 12, 10, 8

Hence,first term (a) = 126, common difference, (d) = 124 - 126

So, $S _{10} = \frac{10}{2} [ 2a + (10 - 1)d ] \:$
$= 5 [2(126) + 9( - 2) ]$
$= 5(252 - 18)$
$= 5 \times 234 = 1170$