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NDA 2019
Exam
NDA
Year
2019
Subject
Mathematics
Chapter
Basics of Mathematics

NDA 2019 · Mathematics · Basics of Mathematics

If x^ 7^x > 7 where x > 0 , then which one of the following is...

If xlog⁡7x>7x^{\log } 7^x > 7 where x>0x > 0, then which one of the following is correct?

Options

  1. Ax∈(0,∞)x \in(0, \infty)
  2. Bx∈(17,7)x \in \left(\frac{1}{7},7\right)
  3. Cx∈(0,17)∪(7,∞)x \in\left(0, \frac{1}{7}\right) \cup(7, \infty)
  4. Dx∈(17,∞)x \in \left(\frac{1}{7}, \infty\right)

Correct answer

C. x∈(0,17)∪(7,∞)x \in\left(0, \frac{1}{7}\right) \cup(7, \infty)

Step-by-step solution

xlog⁡7x>7;x>0x^{\log _7^x} > 7 ; x > 0 taking log both sides ⇒log⁡7x⋅log⁡x>log⁡7\Rightarrow \log _7^x \cdot \log x > \log 7⇒log⁡xlog⁡7⋅log⁡x>log⁡7⇒(log⁡x)2>(log⁡7)2⇒(log⁡x)2−(log⁡7)2>0⇒(log⁡x+log⁡7)(log⁡x−log⁡7)>0⇒x=17,7\begin{aligned} & \Rightarrow \frac{\log x}{\log 7} \cdot \log x > \log 7 \\ & \Rightarrow(\log x)^2 > (\log 7)^2 \\ & \Rightarrow(\log x)^2-(\log 7)^2 > 0 \\ & \Rightarrow(\log x+\log 7)(\log x-\log 7) > 0 \\ & \Rightarrow x=\frac{1}{7}, 7\end{aligned}$<br/><br/>\therefore \quad x \in\left(0, \frac{1}{7}\right) \cup(7, \infty)$
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