Maths-
General
Easy

Question

The extreme values of 4 cos invisible function application open parentheses x to the power of 2 end exponent close parentheses cos invisible function application open parentheses fraction numerator pi over denominator 3 end fraction plus x to the power of 2 end exponent close parentheses cos invisible function application open parentheses fraction numerator pi over denominator 3 end fraction minus x to the power of 2 end exponent close parentheses over R, are

  1. -1, 1  
  2. -2, 2  
  3. -3, 3  
  4. -4, 4  

The correct answer is: -1, 1


    Let f open parentheses x close parentheses equals 4 cos invisible function application left parenthesis x to the power of 2 end exponent right parenthesis cos invisible function application open parentheses fraction numerator pi over denominator 3 end fraction plus x to the power of 2 end exponent close parentheses cos invisible function application open parentheses fraction numerator pi over denominator 3 end fraction minus x to the power of 2 end exponent close parentheses
    equals 2 cos invisible function application open parentheses x to the power of 2 end exponent close parentheses open square brackets cos invisible function application open parentheses fraction numerator 2 pi over denominator 3 end fraction close parentheses plus cos invisible function application open parentheses 2 x to the power of 2 end exponent close parentheses close square brackets

    open square brackets because blank 2 cos invisible function application A cos invisible function application B equals cos invisible function application open parentheses A plus B close parentheses plus cos invisible function application open parentheses A minus B close parentheses close square brackets

    equals 2 cos invisible function application left parenthesis x to the power of 2 end exponent right parenthesis open square brackets negative fraction numerator 1 over denominator 2 end fraction plus cos invisible function application left parenthesis 2 x to the power of 2 end exponent right parenthesis close square brackets

    equals negative cos invisible function application open parentheses x to the power of 2 end exponent close parentheses plus 2 cos invisible function application open parentheses x to the power of 2 end exponent close parentheses cos invisible function application open parentheses 2 x to the power of 2 end exponent close parentheses

    equals negative cos invisible function application open parentheses x to the power of 2 end exponent close parentheses plus cos invisible function application open parentheses 3 x to the power of 2 end exponent close parentheses plus cos invisible function application open parentheses x to the power of 2 end exponent close parentheses

    rightwards double arrow blank f open parentheses x close parentheses equals cos invisible function application left parenthesis 3 x to the power of 2 end exponent right parenthesis ...(i)

    rightwards double arrow blank f to the power of ´ end exponent open parentheses x close parentheses equals negative open square brackets sin invisible function application open parentheses 3 x to the power of 2 end exponent close parentheses close square brackets left parenthesis 6 x right parenthesis

    For extremum, put f to the power of ´ end exponent open parentheses x close parentheses equals 0

    rightwards double arrow blank minus sin invisible function application open parentheses 3 x to the power of 2 end exponent close parentheses open parentheses 6 x close parentheses equals 0

    rightwards double arrow blank x equals 0 comma blank square root of pi

    Put x equals 0 comma blank square root of pi in Eq. (i), we get

    f open parentheses 0 close parentheses equals cos invisible function application open parentheses 0 close parentheses equals 1

    And f open parentheses pi close parentheses equals cos invisible function application left parenthesis 3 pi right parenthesis equals negative 1

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