Maths-
General
Easy

Question

The real number x when added to its inverse gives the minimum value of the sum at x which is equal to

  1. negative 2  
  2. 2  
  3. 1  
  4. negative 1  

The correct answer is: 1


    Let A equals x plus fraction numerator 1 over denominator x end fraction blank rightwards double arrow fraction numerator d A over denominator d x end fraction equals 1 minus fraction numerator 1 over denominator x to the power of 2 end exponent end fraction
    Put fraction numerator d A over denominator d x end fraction equals 0 for maxima or minima

    1 minus fraction numerator 1 over denominator x to the power of 2 end exponent end fraction equals 0 blank rightwards double arrow blank x equals negative 1 comma blank 1

    Again on differentiating w.r.t. x, we get

    fraction numerator d to the power of 2 end exponent A over denominator d x to the power of 2 end exponent end fraction equals fraction numerator 2 over denominator x to the power of 3 end exponent end fraction rightwards double arrow blank open parentheses fraction numerator d to the power of 2 end exponent A over denominator d x to the power of 2 end exponent end fraction close parentheses subscript x equals 1 end subscript equals 2 greater than 0

    therefore A is minimum at x equals 1

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