যদি x > 0 এবং \(\rm x^4 + \frac{1}{x^4} = 142\) হয়, তাহলে \(\rm x^7 + \frac{1}{x^7} \)-এর মান কত?

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SSC CGL 2023 Tier-I Official Paper (Held On: 20 Jul 2023 Shift 2)
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  1. 1561\(\sqrt{14} \)
  2. 1563\(\sqrt{14} \)
  3. 1560\(\sqrt{14} \)
  4. 1562\(\sqrt{14} \)

Answer (Detailed Solution Below)

Option 1 : 1561\(\sqrt{14} \)
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প্রদত্ত

\(\rm x^4 + \frac{1}{x^4} = 142\)

অনুসৃত সূত্র

(x + 1/x)2 = x2 + 1/x2 + 2

x3 + 1/x3 = (x + 1/x)3 - 3(x + 1/x)

গণনা

⇒ x4 + 1/x4 + 2 = (x2 + 1/x2)2

⇒ 142 + 2 = (x2 + 1/x2)2

⇒ (x2 + 1/x 2) = √144 = 12

⇒ (x + 1/x)2 = x2 + 1/x2 + 2

⇒  (x + 1/x)2 = 12 + 2

⇒  (x + 1/x) = √14

⇒ x3 + 1/x3 = (x + 1/x)3 - 3(x + 1/x)

= (√14)3 - 3(√14)

= 14√14 - 3√14 = 11√14

(x7 + 1/x7) = (x4 + 1/x4) ×  (x3 + 1/x3) -  (x + 1/x)

= 142  ×  11√14 -  √14

= 1562√14 - √14

= √14 (1562 - 1)

= 1561√14

উত্তর হল 1561√14

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