हिंदी

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

 

विकल्प

  • (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.

  क्या इस प्रश्न या उत्तर में कोई त्रुटि है?
अध्याय 3: Functions - Exercise 3.6 [पृष्ठ ४३]

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

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