What is the value of (a_9+a_{13}) for the arithmetic progression (21,28,35,\ldots)?
Answer and explanation
Correct answer: 182
Here, the first term is \(a=21\) and the common difference is \(d=28-21=7\). Using \(a_n=a+(n-1)d\), we get \(a_9=21+8\times7=77\) and \(a_{13}=21+12\times7=105\). Therefore, \(a_9+a_{13}=77+105=182\). Option 168 may result from using an incorrect difference instead of \((n-1)d\). Exam tip: while finding the \(n\)th term of an AP, always use \((n-1)d\).
Frequently asked questions
What is the correct answer to this question?
182
Why is this the correct answer?
Here, the first term is \(a=21\) and the common difference is \(d=28-21=7\). Using \(a_n=a+(n-1)d\), we get \(a_9=21+8\times7=77\) and \(a_{13}=21+12\times7=105\). Therefore, \(a_9+a_{13}=77+105=182\). Option 168 may result from using an incorrect difference instead of \((n-1)d\). Exam tip: while finding the \(n\)th term of an AP, always use \((n-1)d\).
Which subject and chapter does this question cover?
This is a Class 9 Mathematics question. Chapter: Sequences and Progressions. Topic: Arithmetic Progression.
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