Maths-
General
Easy

Question

if straight f subscript straight k left parenthesis straight x right parenthesis equals 1 divided by straight k left parenthesis sin to the power of straight k space straight x plus cos to the power of straight k space straight x right parenthesis where straight x element of straight R comma straight k greater or equal than 1 then straight f subscript 4 left parenthesis straight x right parenthesis minus straight f subscript 6 left parenthesis straight x right parenthesis equals

  1. 1 over 6
  2. 1 third
  3. 1 over 9
  4. 1 over 12

The correct answer is: 1 over 12


    Step by step solution:
    Given, f subscript k left parenthesis x right parenthesis space equals space 1 over kopen parentheses sin to the power of k x plus cos to the power of k x close parentheses
    Now, f subscript 4 left parenthesis x right parenthesis space minus space f subscript 6 left parenthesis x right parenthesis equals 1 fourth left parenthesis sin to the power of 4 x plus cos to the power of 4 x right parenthesisspace minus space 1 over 6 left parenthesis sin to the power of 6 x plus cos to the power of 6 x right parenthesis
    equals 1 fourth left parenthesis left parenthesis sin squared x plus cos squared x right parenthesis squarednegative 2 sin squared x space cos squared x right parenthesisnegative 1 over 6 left parenthesis left parenthesis sin squared x plus cos squared x right parenthesis cubed minus3 space sin squared x space cos squared x left parenthesissin squared x plus cos squared x right parenthesis right parenthesis
    equals 1 fourth left parenthesis 1 minus 2 sin squared x space cos squared x right parenthesisnegative 1 over 6 left parenthesis 1 minus 3 sin squared x cos squared x right parenthesis                [ since sin squared theta plus cos squared theta equals 1 ]
    equals 1 fourth minus 1 half sin squared x cos squared xnegative 1 over 6 plus 1 half sin squared x cos squared x
    equals 1 fourth minus 1 over 6equals 1 over 12
    Hence, option(b) is the correct option.

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