Question

when an = 2- 3n find S25​

Answer 1

Step-by-step explanation:

We have ,

n th term = 2 - 3n

→ First term = 2 - 3 × 1 = -1

Second term = 2 - 3 ×2 = -4

Third term = 2 - 3 ×3 = -7

Clearly the terms are in AP .

We have , common difference = A2 - A1 = - 4 - ( -1) = - 4 + 1 = -3 .

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

→ S 25 = 25/2 { 2 × ( - 1 ) + ( 25 - 1 ) (-3) }

→ S 25 = 25/2 { -2 +24(-3)}

→ S 25 = 25 / 2 { -74 }

→ S 25 = 25 × ( - 37 )

→ S 25 = - 925

Hence the required answer is - 925