Answer :

Given A.P. Series: –1, 4, 9, 14, ......

Since the series is an A.P. series hence between every consecutive terms there is a common difference (d)

We know that the nth term in an A.P. series is given by

tn = a + (n–1)d


tn : Represents the nth term of an A.P.

a : Represents the first term of the A.P. series

d : Represents the common difference

n : Represents the position of nth term of the A.P.

So according to the problem:

129 = -1 + (n-1) × 5

130 = (n-1) × 5

n-1 = 26

Answer: The value of n = 27

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