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

Here, first term (a) = 126,

common difference (d) = 124 – 126 = – 2

∴ `S_10 = 10/2 [2a + (10 - 1)d]`

∵ `S_n = n/2 [2a + (n - 1)d]`

= 5{2(126) + 9(–2)}

= 5(252 – 18)

= 5 × 234

= 1170