Question

Which term of the A.P. 3, 15, 27, 39, ... will be 120 more than its 21st term?​

Answer 1

Answer:

In A.P 3, 15, 27, 39, the term is 243 which is 120 more than required term 363.

Solution:

Common difference of series = (n+1) – n

We get 15 – 3, 27 – 15……  

Common difference = 12

First term of series = 3.  

Therefore, the 21st term of the series is  

Required term after adding 120 = 120 + 243 = 363.

Let  

First value of the series is a = 3

Common difference = 12  

term is 363.  

Value of n:  

Tn = 3 + 12(n-1)

363 = 3 + 12(n-1)

n = 31