NCERT Solutions
Class 11 Maths
Sequences and Series

Ex.9.2 Q.16
Between 1 and 31, m numbers have been inserted in such a way that the resulting sequence is an A.P.
and the ratio of 7th and (m – 1) th numbers is 5: 9. Find the value of m.
Let A1, A2, ... Am be m numbers such that 1, A1, A2, ... Am, 31 is an A.P.
Here, a = 1, b = 31, n = m + 2
So, 31 = 1 + (m + 2 – 1) (d)
30 = (m + 1) d
d = 30 ÷ (m + 1) ………... (1)
A1 = a + d
A2 = a + 2d
A3 = a + 3d
………………
A7 = a + 7d
Am–1 = a + (m – 1) d
According to the given condition,
(a + 7d) ÷ {a + (m - 1) d} =
=> [1 + 7{30 ÷ (m + 1)}] ÷ {1 + (m - 1) × (30 ÷ (m + 1))} =
[From equation 1]
=> [m + 1 + 7 × 30] ÷ [m + 1 + 30(m - 1)] =
=> [m + 1 + 210] ÷ [m + 1 + 30m - 30] =
=> [m + 211] ÷ [31m - 29] = 59
=> 9[m + 211] = 5[31m - 29]
=> 9m + 1899 = 155m - 145
=> 155m – 9m = 1899 + 145
=> 146m = 2044
=> m =
=> m = 14
Thus, the value of m is 14.