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Revision Notes: Sequence & Series

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FAQs on Revision Notes: Sequence & Series

1. What's the difference between arithmetic progression and geometric progression?
Ans. An arithmetic progression (AP) has a constant difference between consecutive terms, while a geometric progression (GP) has a constant ratio. In AP, you add a fixed value each time; in GP, you multiply by a fixed value. AP examples: 2, 5, 8, 11. GP examples: 2, 6, 18, 54. Understanding both progression types is essential for JEE Main and Advanced problems.
2. How do I find the sum of an infinite geometric series without getting confused?
Ans. The sum of an infinite geometric series equals a/(1-r), where 'a' is the first term and 'r' is the common ratio, but only when |r| < 1. If |r| ≥ 1, the series diverges and has no finite sum. Students often forget the convergence condition, causing wrong answers. Always check whether the common ratio's absolute value is less than one before applying the formula.
3. Why do some sequences have limits while others don't in sequence and series problems?
Ans. A sequence converges to a limit when its terms approach a specific value as n increases indefinitely; it diverges otherwise. For example, 1/n converges to 0, but n² diverges to infinity. Recognizing convergent and divergent behaviour helps determine whether series formulas apply. Monotonic sequences bounded above or below tend to converge, which is crucial for JEE problem-solving strategies.
4. What's the trick to solving questions about the nth term and sum formulas in arithmetic progressions?
Ans. For arithmetic progressions, the nth term formula is aₙ = a + (n-1)d, and sum of first n terms is Sₙ = n/2[2a + (n-1)d]. Memorise both formulas separately-students often mix them up. Once you identify the first term 'a' and common difference 'd' from the question, substitute directly. Practice with EduRev's MCQ tests and worksheets to build speed and accuracy for competitive exams.
5. How do harmonic progressions and their relationship with arithmetic progressions work in exams?
Ans. A harmonic progression (HP) is formed when reciprocals of terms create an arithmetic progression. If 1/a, 1/b, 1/c form an AP, then a, b, c form an HP. The harmonic mean formula for two numbers x and y is 2xy/(x+y). HP problems frequently appear in JEE Advanced; solving them requires converting to AP, finding terms, then converting back to original form.
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