मराठी

If F(X) = Cos (Loge X), Then F ( 1 X ) F ( 1 Y ) − 1 2 { F ( X Y ) + F ( X Y ) } is Equal To(A) Cos (X − Y) (B) Log (Cos (X − Y)) (C) 1 (D) Cos (X + Y) - Mathematics

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प्रश्न

If f(x) = cos (loge x), then \[f\left( \frac{1}{x} \right)f\left( \frac{1}{y} \right) - \frac{1}{2}\left\{ f\left( xy \right) + f\left( \frac{x}{y} \right) \right\}\] is equal to

 

पर्याय

  • (a) cos (x − y)

  • (b) log (cos (x − y))

  • (c) 1

  • (d) cos (x + y)

     
  • (e) 0

MCQ

उत्तर

Given:

\[f\left( x \right) = \cos\left( \log_e x \right)\]
\[\Rightarrow f\left( \frac{1}{x} \right) = \cos\left( \log_e \left( \frac{1}{x} \right) \right)\]
\[ \Rightarrow f\left( \frac{1}{x} \right) = \cos\left( - \log_e \left( x \right) \right)\]
\[ \Rightarrow f\left( \frac{1}{x} \right) = \cos\left( \log_e \left( x \right) \right)\]
Similarly,
\[f\left( \frac{1}{y} \right) = \cos\left( \log_e y \right)\]
Now,
\[f\left( xy \right) = \cos\left( \log_e xy \right) = \cos\left( \log_e x + \log_e y \right)\]
  and
 
\[f\left( \frac{x}{y} \right) = \cos\left( \log_e \frac{x}{y} \right) = \cos\left( \log_e x - \log_e y \right)\]
\[\Rightarrow f\left( \frac{x}{y} \right) + f\left( xy \right) = \cos\left( \log_e x - \log_e y \right) + \cos\left( \log_e x + \log_e y \right)\]
\[ \Rightarrow f\left( \frac{x}{y} \right) + f\left( xy \right) = 2\cos\left( \log_e x \right)\cos\left( \log_e y \right)\]
\[ \Rightarrow \frac{1}{2}\left[ f\left( \frac{x}{y} \right) + f\left( xy \right) \right] = \cos\left( \log_e x \right)\cos\left( \log_e y \right)\]
\[\Rightarrow f\left( \frac{1}{x} \right)f\left( \frac{1}{y} \right) - \frac{1}{2}\left\{ f\left( xy \right) + f\left( \frac{x}{y} \right) \right\} = \cos\left( \log_e x \right)\cos\left( \log_e y \right) - \cos\left( \log_e x \right)\cos\left( \log_e y \right) = 0\]
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Notes

Disclaimer: The question in the book has some error, so none of the options are matching with the solution. The solution is created according to the question given in the book.

  या प्रश्नात किंवा उत्तरात काही त्रुटी आहे का?
पाठ 3: Functions - Exercise 3.6 [पृष्ठ ४३]

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आरडी शर्मा Mathematics [English] Class 11
पाठ 3 Functions
Exercise 3.6 | Q 18 | पृष्ठ ४३

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