Question

Find the sum of last ten terms of the AP 8,10,12,14, ....,126​

Answer 1

Answer:

REVERSE THE SERIES

first term of new AP is = 126

common difference = -2

Formula used:-

Sn =n/2 {2a +(n-1)d

S10=10/2{2 × 126 +(10-1)(-2)}

=5{252 -18}  

=1170

Answer 2

The sum of last ten terms of the AP: 8,10,12,...,126 is same as the sum of first ten terms of the AP: 126,124,122,...,8.

Therefore, a=126,d=−2 and n=10.

Now, S  10  =  10/2 [2×126+(10−1)×−2]

⇒S  10 =5×[252−18]=1170