Medium MCQ +4 / -1 PYQ · JEE Mains 2020

If ${3^{2\sin 2\alpha - 1}}$, 14 and ${3^{4 - 2\sin 2\alpha }}$ are the first three terms of an A.P. for some $\alpha$, then the sixth terms of this A.P. is:

  1. A 66 Correct answer
  2. B 81
  3. C 65
  4. D 78

Solution

Given that<br><br>${3^{4 - \sin 2\alpha }} + {3^{2\sin 2\alpha - 1}} = 28$<br><br>Let ${3^{2\sin 2\alpha }}$ = t<br><br>$\Rightarrow$ ${{81} \over t} + {t \over 3} = 28$<br><br>$\Rightarrow$t = 81, 3<br><br>$\therefore$ ${3^{2\sin 2\alpha }}$ = 3<sup>1</sup>, 3<sup>4</sup><br><br>$\sin 2\alpha = {1 \over 2}$, 2 (rejected)<br><br>First term a = ${3^{2\sin 2\alpha -1}}$ = 3<sup>0</sup> <br><br>$\Rightarrow$ a = 1<br><br>Given Second term = 14<br><br>$\therefore$ Common difference d = 13<br><br>${T_6} = a + 5d$<br><br>${T_6} = 1 + 5 \times 13$<br><br>${T_6} = 66$

About this question

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

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