Solution:
Let Sn, and S' be the sums of n terms of two AP's and T11 and T11, be the respective 11th term, then
(given)
Now, put n=21, we get
If the ratio of the sum of n terms of two AP's be (7n + 1):(4n + 27), then the ratio of their 11th terms will be
Let Sn, and S' be the sums of n terms of two AP's and T11 and T11, be the respective 11th term, then
(given)
Now, put n=21, we get