Find the value of x, if log2 (5.2^x + 1), log4(2^(1–x) + 1) and 1 are in A.P. -Maths 9th

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Answer :

(b) 1 – log25 Given, log2 (5.2x + 1), log4 (21– x + 1), 1 are in A.P. ⇒ log2 (5.2x + 1) + 1 = 2 log4 (21 – x + 1) ⇒ log2 (5.2x + 1) + log22 = 2 log22 (21-x + 1)⇒ log2 (5.2x + 1).2 = 2 x \(rac12\)log2 (21-x + 1)\(\big(\because\, ext{log}_{a^n}x=rac{1}{n} ext{log}_ax\big)\)⇒ log2 (10.2x + 2) = log2 (21-x + 1)⇒ 10.2x + 2 = 21 – x + 1 ⇒ 10.2x + 2 = \(rac{2}{2^x}\) + 1Let 2x = a, then10. a + 2 = \(rac{2}{a}\) + 1 ⇒ 10a + 1 = \(rac{2}{a}\) ⇒ 10 a2 + a – 2 = 0⇒ (5 a – 2) (2a + 1) = 0 ⇒ a = \(rac{2}{5}\) ⇒ 2x = \(rac{2}{5}\)\(\big(\because\,2^x>0, ext{reject}\,a=-rac{1}{2}\big)\)⇒ log 2x = log \(rac{2}{5}\)⇒ x log2 2 = log2 2 – log2 5 ⇒ \(x\) = 1 – log2 5.

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