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)thnumbers 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
∴ 31 = 1 + (m + 2 – 1) (d)
⇒ 30 = (m + 1) d
A1 = a + d
A2 = a + 2d
A3 = a + 3d …
∴ A7 = a + 7d
Am–1 = a + (m – 1) d
According to the given condition,
Thus, the value of m is 14.
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