If the ratio of the sum of the first n terms of two A.Ps is (7n + 1) : (4n + 27), then find the ratio of their 9th terms.
Let the first terms be a and a' and d and d' be their respective common differences.
To get ratio of 9th terms, replacing (n–1)/2 = 8
n = 17
t9/t9' = (a + 8d)/(a' + 8d')