Question

Question 30
Find the sum of last ten terms of the AP 8, 10, 12,…., 126.

Answer 1

To find the sum of last ten terms, we write the given AP in reverse order.
i.e, 126, 124, 122, …… 12, 10, 8
Here, first term, a = 126, common difference, d = 124 – 126 = -2
$\therefore S_{10}=\frac{10}{2}[2a+(10-1\left)d\right][\because S_n=\frac{n}{2}[2a+(n-1)d\left]\right]$
$=5\left\{2\right(126)+9(-2\left)\right\}=5(252-18)$
$=5\times 234=1170$