Maths-
General
Easy

Question

Suppose that f is a polynomial of degree 3 and that f to the power of ´ ´ end exponent left parenthesis x right parenthesis not equal to 0 at any of the stationary point. Then

  1.  f.has exactly one stationary point    
  2. f must have no stationary point    
  3. f must have exactly two stationary points    
  4. f has either zero or two stationary point    

The correct answer is: f has either zero or two stationary point


    y equals fraction numerator log invisible function application x over denominator x end fraction
    rightwards double arrow fraction numerator d y over denominator d x end fraction equals negative fraction numerator 1 over denominator x to the power of 2 end exponent end fraction log invisible function application x plus fraction numerator 1 over denominator x end fraction fraction numerator 1 over denominator x end fraction
    equals fraction numerator 1 over denominator x to the power of 2 end exponent end fraction open parentheses 1 minus log invisible function application x close parentheses equals 0
    fraction numerator d y over denominator d x end fraction equals 0 rightwards double arrow log invisible function application x equals 1 o r blank x equals e
    For x less than e rightwards double arrow log invisible function application x less than 1
    and x greater than e rightwards double arrow log invisible function application x greater than 1
    At x equals e comma fraction numerator d y over denominator d x end fraction changes sign from +ve to –ve and hence y is maximum at x equals e and its value is
    fraction numerator log invisible function application e over denominator e end fraction equals e to the power of negative 1 end exponent
    Also, f to the power of ´ ´ end exponent open parentheses x close parentheses equals 2 m left parenthesis x minus alpha right parenthesis, in which case f to the power of ´ ´ end exponent open parentheses alpha close parentheses equals 0. But we are told that the 2nd derivative is non-zero at critical point. Hence, there must be either 0 or 2 critical points

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