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2013

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- A.
9
- B.
15
- C.
18
- D.
27
Attempted by 12 students.
Show answer & explanation
Correct answer: C
Let y = x + \frac{1}{x}. Then, squaring both sides:
y^2 = x^2 + 2 + \frac{1}{x^2} \Rightarrow x^2 + \frac{1}{x^2} = y^2 - 2
Now square again:
(x^2 + \frac{1}{x^2})^2 = x^4 + 2 + \frac{1}{x^4} \Rightarrow x^4 + \frac{1}{x^4} = (y^2 - 2)^2 - 2
Given x^4 + \frac{1}{x^4} = 47:
(y^2 - 2)^2 - 2 = 47
Solve for y^2:
(y^2 - 2)^2 = 49 \Rightarrow y^2 - 2 = \pm 7
So y^2 = 9 or y^2 = -5. Since y^2 \geq 0, y^2 = 9 \Rightarrow y = \pm 3
Now, x^3 + \frac{1}{x^3} = y^3 - 3y
If y = 3: 3^3 - 3(3) = 27 - 9 = 18
If y = -3: (-3)^3 - 3(-3) = -27 + 9 = -18
Thus, x^3 + \frac{1}{x^3} = \pm 18
हिन्दी उत्तर:
माना y = x + \frac{1}{x}. तब दोनों तरफ वर्ग करने पर:
y^2 = x^2 + 2 + \frac{1}{x^2} \Rightarrow x^2 + \frac{1}{x^2} = y^2 - 2
अब फिर से वर्ग करें:
(x^2 + \frac{1}{x^2})^2 = x^4 + 2 + \frac{1}{x^4} \Rightarrow x^4 + \frac{1}{x^4} = (y^2 - 2)^2 - 2
दिया गया है x^4 + \frac{1}{x^4} = 47:
(y^2 - 2)^2 - 2 = 47
y^2 के लिए हल करें:
(y^2 - 2)^2 = 49 \Rightarrow y^2 - 2 = \pm 7
तो y^2 = 9 या y^2 = -5. चूँकि y^2 \geq 0, इसलिए y^2 = 9 \Rightarrow y = \pm 3
अब, x^3 + \frac{1}{x^3} = y^3 - 3y
यदि y = 3: 3^3 - 3(3) = 27 - 9 = 18
यदि y = -3: (-3)^3 - 3(-3) = -27 + 9 = -18
इसलिए, x^3 + \frac{1}{x^3} = \pm 18