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General
Easy

Question

If f left parenthesis a right parenthesis equals 2 comma f to the power of straight prime left parenthesis a right parenthesis equals 1 comma g left parenthesis a right parenthesis equals negative 1 comma g to the power of factorial left parenthesis a right parenthesis equals 2, then the value of Lim subscript x not stretchy rightwards arrow a end subscript space fraction numerator g left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f left parenthesis x right parenthesis over denominator x minus a end fraction is

  1. -5
  2. 1 fifth
  3. 5
  4. none of these

hintHint:

We can apply L'Hopital's rule, also commonly spelled L'Hospital's rule, whenever direct substitution of a limit yields an indeterminate form. This means that the limit of a quotient of functions (i.e., an algebraic fraction) is equal to the limit of their derivatives.
In this question, we have to find value of Lim subscript x not stretchy rightwards arrow a end subscript space fraction numerator g left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f left parenthesis x right parenthesis over denominator x minus a end fraction .

The correct answer is: 5


    If f left parenthesis a right parenthesis equals 2 comma f to the power of straight prime left parenthesis a right parenthesis equals 1 comma g left parenthesis a right parenthesis equals negative 1 comma g to the power of factorial left parenthesis a right parenthesis equals 2, then the value of Lim subscript x not stretchy rightwards arrow a end subscript space fraction numerator g left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f left parenthesis x right parenthesis over denominator x minus a end fraction
    Lim subscript x not stretchy rightwards arrow a end subscript space fraction numerator g left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f left parenthesis x right parenthesis over denominator x minus a end fraction
    We first try substitution:
    Lim subscript x not stretchy rightwards arrow a end subscript space fraction numerator g left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f left parenthesis x right parenthesis over denominator x minus a end fraction = space fraction numerator g left parenthesis a right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f left parenthesis a right parenthesis over denominator x minus a end fraction space equals fraction numerator negative 1 cross times 2 plus 1 cross times 2 right parenthesis over denominator a minus a end fraction space equals 0 over 0
    Since the limit is in the form 0 over 0, it is indeterminate—we don’t yet know what is it. We need to do some work to put it in a form where we can determine the limit.
    Lim subscript x not stretchy rightwards arrow a end subscript space fraction numerator g left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f left parenthesis x right parenthesis over denominator x minus a end fraction = Lim subscript x not stretchy rightwards arrow a end subscript space fraction numerator g apostrophe left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f apostrophe left parenthesis x right parenthesis over denominator begin display style fraction numerator d over denominator d x end fraction end style left parenthesis x right parenthesis end fraction   { apply L'Hopital's rule }
     G i v e n space t h a t space space f left parenthesis a right parenthesis equals 2 comma f to the power of straight prime left parenthesis a right parenthesis equals 1 comma g left parenthesis a right parenthesis equals negative 1 comma g to the power of factorial left parenthesis a right parenthesis equals 2,
    Lim subscript x not stretchy rightwards arrow a end subscript space fraction numerator g apostrophe left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f apostrophe left parenthesis x right parenthesis over denominator begin display style fraction numerator d over denominator d x end fraction end style left parenthesis x right parenthesis end fraction space space equals Lim subscript x not stretchy rightwards arrow a end subscript space g apostrophe left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f apostrophe left parenthesis x right parenthesis space
Lim subscript x not stretchy rightwards arrow a end subscript space g apostrophe left parenthesis x right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f apostrophe left parenthesis x right parenthesis space equals space g apostrophe left parenthesis a right parenthesis f left parenthesis a right parenthesis minus g left parenthesis a right parenthesis f apostrophe left parenthesis a right parenthesis space equals space 2 cross times 2 space plus 1 cross times 1 space equals 5

    We can only apply the L’Hospital’s rule if the direct substitution returns an indeterminate form, that means 0 over 0 orfraction numerator plus-or-minus infinity over denominator plus-or-minus infinity end fraction.

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