ClassesClass 9MathsNCERTSequences and ProgressionsExercise 8.1Q 5
QUESTION 5 Easy

A sequence is given by the recursive rule \(t_1 = −5, t_{n+1} = t_n + 3\) for \(n ≥ 1\). Find the first five terms of the sequence. Is 52 a term of this sequence? If so, which term is it?

SOLUTION

1
Note down what is given
\(t_1 = −5\)
\(t_{n+1} = t_n + 3\)
2
Find the first five terms
\(t_1 = −5\)
\(t_2 = t_1 + 3 = −5 + 3 = −2 \)
\(t_3 = t_2 + 3 = −2 + 3 = 1\)
\(t_4 = t_3 + 3 = 1 + 3 = 4\)
\(t_5 = t_4 + 3 = 4 + 3 = 7\)
The first five terms are: −5, −2, 1, 4, 7
3
Is 52 a term of the sequence?
Here,
first term, \(a = −5\), and common difference, \(d = 3\)
So, the general term is given by:
\(t_n = a + (n − 1)d\)
⇒ \(t_n = −5 + 3(n − 1)\)
⇒ \(t_n = 3n − 8\)
Set \(t_n = 52\)
⇒ \(3n − 8 = 52\)
⇒ \(3n = 60\)
⇒ \(n = 20\)
Since \(n\) is a positive number, \(52\) is a term of the sequence. It is the \(20^{th}\) term.
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Final Answer : First five terms: −5, −2, 1, 4, 7
Yes, 52 is a term of the sequence. It is the \(20^{th}\) term.

Concept Note

A recursive sequence is defied using the previous terms.
Example: \(t_{n−1} = t_n + 3\)
Each term is obtained from the preceding term.