Maths-
General
Easy

Question

If x y equals c to the power of 2 end exponent, then minimum value of a x plus b y is

  1. c square root of a b end root  
  2. 2 c square root of a b end root  
  3. negative c square root of a b end root  
  4. negative 2 c square root of a b end root  

The correct answer is: 2 c square root of a b end root


    Given curve is
    x y equals c to the power of 2 end exponent blank rightwards double arrow y equals fraction numerator c to the power of 2 end exponent over denominator x to the power of 2 end exponent end fraction

    Let f open parentheses x close parentheses equals a x plus b y equals a x plus fraction numerator b c to the power of 2 end exponent over denominator x to the power of 2 end exponent end fraction

    On differentiating w.r.t. x, we get

    f to the power of ´ end exponent open parentheses x close parentheses equals a minus fraction numerator b c to the power of 2 end exponent over denominator x to the power of 2 end exponent end fraction

    For a maximum or minima, put f to the power of ´ end exponent open parentheses x close parentheses equals 0

    rightwards double arrow blank a x to the power of 2 end exponent minus b c to the power of 2 end exponent equals 0

    rightwards double arrow blank x to the power of 2 end exponent equals fraction numerator b c to the power of 2 end exponent over denominator a end fraction blank rightwards double arrow blank x equals plus-or-minus c square root of fraction numerator b over denominator a end fraction end root

    Again on differentiating, we get f to the power of ´ ´ end exponent open parentheses x close parentheses equals fraction numerator 2 b c to the power of 2 end exponent over denominator x to the power of 3 end exponent end fraction

    At6 x equals c square root of fraction numerator b over denominator a end fraction end root comma blank f to the power of ´ ´ end exponent open parentheses x close parentheses greater than 0

    therefore blank f open parentheses x close parentheses is minimum at x equals c square root of fraction numerator b over denominator a end fraction end root

    The minimum value at x equals c square root of fraction numerator b over denominator a end fraction end root is

    therefore blank f open parentheses c square root of fraction numerator b over denominator a end fraction end root close parentheses equals a. c square root of fraction numerator b over denominator a end fraction end root plus fraction numerator b c to the power of 2 end exponent over denominator c end fraction. blank square root of fraction numerator a over denominator b end fraction end root

    equals fraction numerator a b c plus a b c over denominator square root of a b end root end fraction equals fraction numerator 2 a b c over denominator square root of a b end root end fraction equals 2 c square root of a b end root

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