If Sn denotes the sum of n terms of an AP and if S1= 6 , S7 = 105 , then show that Sn : S(n-3) = (n+3) : (n-3)

Let the first term and common difference of the given A.P. be a and d respectively.

 

Then S1 = a1 = a = 6

 

 

Hence Sn : Snā€“3 = (n + 3) : (n ā€“ 3)

  • 19
What are you looking for?