In an online course, (4) videos are watched on the first day and (2) more videos each next day. How many videos are watched on the (9)th day?
Answer and explanation
Correct answer: 20
The numbers of videos watched each day form an AP with first term \(a=4\), common difference \(d=2\), and \(n=9\). Thus, \(a_9=a+(n-1)d=4+(9-1)\times2=20\). Therefore, 20 is correct. The value 18 would represent the 8th-day term, not the 9th-day term. Exam tip: use \(n-1\), not \(n\), in the formula \(a_n=a+(n-1)d\).
Frequently asked questions
What is the correct answer to this question?
20
Why is this the correct answer?
The numbers of videos watched each day form an AP with first term \(a=4\), common difference \(d=2\), and \(n=9\). Thus, \(a_9=a+(n-1)d=4+(9-1)\times2=20\). Therefore, 20 is correct. The value 18 would represent the 8th-day term, not the 9th-day term. Exam tip: use \(n-1\), not \(n\), in the formula \(a_n=a+(n-1)d\).
Which subject and chapter does this question cover?
This is a Class 10 Mathematics question. Chapter: Arithmetic Progressions (AP). Topic: Word problems based on APs.
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