Q. 184.3( 10 Votes )

If the ratio between the sums of n terms of two arithmetic progressions is (7n + 1) : (4n + 27), find the ratio of their 11th terms.

Answer :

Given: Ratio of sum of nth terms of 2 AP’s


To Find: Ratio of their 11th terms


Let us consider 2 AP series AP1 and AP2.


Putting n = 1, 2, 3… we get AP1 as 8, 15 22… and AP2 as 31, 35, 39….


So, a1 = 8, d1 = 7 and a2 = 31, d2 = 4


For AP1


S6 = 8 + (11 - 1)7 = 87


For AP2


S6 = 31 + (11 - 1)4 = 81


Required ratio =


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