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)]
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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 .