Maths-
General
Easy

Question

Lt subscript y not stretchy rightwards arrow 0 end subscript space fraction numerator sin begin display style space end style left parenthesis a plus b x right parenthesis minus sin space left parenthesis a minus b x right parenthesis over denominator x end fraction

  1. 2bcos a
  2. 2bsin a
  3. 2cos a
  4. 2sin a

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 Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator sin begin display style space end style left parenthesis a plus b x right parenthesis minus sin space left parenthesis a minus b x right parenthesis over denominator x end fraction.

The correct answer is: 2bcos a


    Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator sin begin display style space end style left parenthesis a plus b x right parenthesis minus sin space left parenthesis a minus b x right parenthesis over denominator x end fraction
    We first try substitution:
    Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator sin begin display style space end style left parenthesis a plus b x right parenthesis minus sin space left parenthesis a minus b x right parenthesis over denominator x end fraction = Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator sin begin display style space end style left parenthesis a plus 0 x right parenthesis minus sin space left parenthesis a minus 0 x right parenthesis over denominator 0 end fraction = 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.
    Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator sin begin display style space end style left parenthesis a plus b x right parenthesis minus sin space left parenthesis a minus b x right parenthesis over denominator x end fraction     (sin (a + b) = sin a cos b + cos a sin b , sin (a - b) = sin a cos b - cos a sin b)
    Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator sin space a space cos space b x space plus space cos space a space sin space b x minus sin space a space cos space b x space plus space cos space a space sin space b x over denominator x end fraction
    Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator 2 cos space a space sin space b x over denominator x end fraction cross times b over b     (space w e space k n o w space t h a t space comma space Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator sin space x over denominator x end fraction space equals space 1)
    2 b cos space a space cross times space Lt subscript x not stretchy rightwards arrow 0 end subscript space fraction numerator sin space b x over denominator b x end fraction
    or, 2 b space cos space a

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

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