Q11 of 46 Page 1

The 4th term of an AP is twice its 18th term. If its 6th term is - 8, then find the sum of its first 20 terms.

Let the a be first term and d be the common difference of AP


 


Given


 


a4 = 2a18


 


using nth term formula i.e. an = a + (n - 1)d


 


a + 3d = 2(a + 17d)


 


a + 3d = 2a + 34d


 


a + 31d = 0


 


a = - 31d [1]


 


Also,


 


a6 = - 8


 


a + 5d = - 8


 


- 31d + 5d = - 8 [using 1]


 


- 26d = - 8


 



 



 


Also sum of first n terms of an AP is


 



 



 



 



 

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