Medium MCQ +4 / -1 PYQ · JEE Mains 2022

$$x = \sum\limits_{n = 0}^\infty {{a^n},y = \sum\limits_{n = 0}^\infty {{b^n},z = \sum\limits_{n = 0}^\infty {{c^n}} } } $$, where a, b, c are in A.P. and |a| < 1, |b| < 1, |c| < 1, abc $\ne$ 0, then :

  1. A x, y, z are in A.P.
  2. B x, y, z are in G.P.
  3. C ${1 \over x}$, ${1 \over y}$, ${1 \over z}$ are in A.P. Correct answer
  4. D ${1 \over x}$ + ${1 \over y}$ + ${1 \over z}$ = 1 $-$ (a + b + c)

Solution

<p>$$x = \sum\limits_{n = 0}^\infty {{a^n} = {1 \over {1 - a}};\,y = \sum\limits_{n = 0}^\infty {{b^n} = {1 \over {1 - b}};\,z = \sum\limits_{n = 0}^\infty {{c^n} = {1 \over {1 - c}}} } } $$</p> <p>Now,</p> <p>a, b, c $\to$ AP</p> <p>1 $-$ a, 1 $-$ b, 1 $-$ c $\to$ AP</p> <p>${1 \over {1 - a}},\,{1 \over {1 - b}},\,{1 \over {1 - c}} \to HP$</p> <p>x, y, z $\to$ HP</p> <p>$\therefore$ ${1 \over x},{1 \over y},{1 \over z} \to AP$</p>

About this question

Subject: Mathematics · Chapter: Sequences and Series · Topic: Arithmetic Progression

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