This is the Question Number 5 of Exercise 11 C of RS Aggarwal Solution an Important Question of Arithmetic Progression For ICSE & CBSE Board Students
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.
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Which term of the AP 5 , 9, 13, 17, …..is 81? Also find the sum ( 5+9+13+17+…..+81)
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first term,a=5 ,
common difference,d=4
Let 81 be the nth term
Tn=a+(n-1)d
81=5+(n-1)4
(n-1)4=76
n-1=19
n=20
Hence 81 is the 20th term
Sn=n/2{2a+(n-1)d}
Sum of the 20 terms = 20/2{2*5+19*4} = 10(10+76) = 860