Question

Find the sum of 25 terms of the A.P. 8 + 5 + 2 + .... .

Answer 1

a = 8

d = 5 − 8

= −3

n = 25

`S_n = n/2 [2a + (n - 1)d]`

`S_25 = 25/2 [2(8) + (25 - 1)(-3)]`

= `25/2[16 + 24(−3)]`

= `25/2[16 + (-72)]`

= `25/2 [-56]`

= `25 xx (-56)/2`

= 25 × (−28)

= − 700