In this Question the last term of AP is given and you have to find which term of the AP this must be and also the sum of the Given AP

This is the Important question based on Arithmetic progression Chapter of R.S Aggarwal book for ICSE & CBSE Board.

This is the Question Number 6 Of Exercise 11 C of RS Aggarwal Solution

Deepak BoraNewbie

# Which term of the AP 15 ,11, 7,…..is (-13)? Also find the sum [15+11+7+…..+(-13)]

Share

This answer was edited.AP : 15 , 11 , 7…..

a= 15 , d= -4 an = -13

an= a+ (n-1) d

-13 = 15 + (n-1) -4

-28 = -4n + 4

-32 = -4n

So , n = 8

For Sum,

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

Sn = 8/2 [ 30 + (7) (-4) ]

Sn = 4 [ 30 -28 ]

Sn = 4 x 2

Finally , Sn = 8 .