Chapter 8
Predicting What Comes Next: Exploring Sequences and Progressions
CBSE Class 9
Mathematics Solutions
Educart Mathematics class Class 9 NCERT Exemplar cover
Question:

A sequence is given by the recursive rule t1=5t_1 = -5, tn+1=tn+3t_{n+1} = t_{n+3} for n1n \geq 1. Find the first five terms of the sequence. Is 52 a term of this sequence? If so, which term is it?                                                                           [Page No. 179]

Answer: Verified

Recursive sequence: t1=5,tn+1=tn+3t_1 = -5, t_{n+1} = t_n + 3

First five terms:

t1=5t_1 = -5

t2=2t_2 = -2

t3=1t_3 = 1

t4=4t_4 = 4

t5=7t_5 = 7

First five terms: -5, -2, 1, 4, 7

To check whether 52 is a term:

This is an arithmetic sequence with first term -5 and common difference 3.

tn=5+(n1)×3t_n = -5 + (n - 1) \times 3

52=5+3(n1)52 = -5 + 3(n - 1)

57=3(n1)57 = 3(n - 1)

19=n119 = n - 1

n=20n = 20

So, 52 is a term of the sequence, and it is the 20th20^{th} term.

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