Question

If general term of an AP is (3 - 4n), find its Sn.​

Answer 1

General term = (3-4n) ..... (i)

Putting n = 1 , in eq.(i) ,

3 - 4 × 1

= 3 - 4

= - 1

Therefore, the first term(a) is -1 .

Putting n = 2 in eq. (i) ,

3 - 4 × 2

= 3 - 8

= - 5

Therefore, the second term is -5.

Common difference(d) = second term - first term

= -5 - (-1)

= -5 + 1

= - 4

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

= n/2 ( 2 × -1 + (n-1) -4)

= n/2 ( -2 - 4n + 4 )

= n/2 ( 2- 4n )

= n - 2n^2