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Class 9MathematicsSequences and Progressionsnth termHardLevel 58
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If (a_n=4n+9), what is (a_{5p-1})?

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Answer and explanation

Correct answer: \(20p+5\)

Given \(a_n=4n+9\), substitute the complete index \(5p-1\) for \(n\): \(a_{5p-1}=4(5p-1)+9=20p-4+9=20p+5\). Therefore, \(20p+5\) is correct. The option \(20p+9\) would result from incorrectly ignoring the \(-1\) in the index. Exam tip: when an index is an expression, substitute the whole expression using brackets.

Related tags

Sequences And ProgressionsNth TermAlgebraic SubstitutionLinear SequenceMathematics Class 9

Frequently asked questions

What is the correct answer to this question?

\(20p+5\)

Why is this the correct answer?

Given \(a_n=4n+9\), substitute the complete index \(5p-1\) for \(n\): \(a_{5p-1}=4(5p-1)+9=20p-4+9=20p+5\). Therefore, \(20p+5\) is correct. The option \(20p+9\) would result from incorrectly ignoring the \(-1\) in the index. Exam tip: when an index is an expression, substitute the whole expression using brackets.

Which subject and chapter does this question cover?

This is a Class 9 Mathematics question. Chapter: Sequences and Progressions. Topic: nth term.

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  4. 4What will be the 12th term in the sequence (3, 5, 9, 15, 23, ...)?
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