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Question

Let f open parentheses x close parentheses equals x to the power of 3 end exponent minus 6 x to the power of 2 end exponent plus 12 x minus 3. Then at x equals 2 comma blank f left parenthesis x right parenthesis has

  1. A maximum  
  2. A minimum  
  3. Both a maximum and a minimum  
  4. Neither a maximum nor a minimum  

The correct answer is: Neither a maximum nor a minimum


    We have,
    f open parentheses x close parentheses equals x to the power of 3 end exponent minus 6 x to the power of 2 end exponent plus 12 x minus 3

    rightwards double arrow f to the power of ´ end exponent open parentheses x close parentheses equals 3 x to the power of 2 end exponent minus 12 x plus 12 comma blank f to the power of ´ end exponent open parentheses x close parentheses equals 6 x minus 12 and f to the power of ´ ´ ´ end exponent open parentheses x close parentheses equals 6

    We observe that f to the power of ´ end exponent open parentheses 2 close parentheses equals 0 comma blank f to the power of ´ ´ end exponent open parentheses 2 close parentheses equals 0 but f ´ ´ ´ left parenthesis 2 right parenthesis not equal to 0

    So, x equals 2 is neither a point of local maximum nor a point of local minimum. In fact, it is a point of inflexion

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