

InterviewSolution
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Let f : R → R be a continuously differentiable function such that f(2) = 6 and \({\rm{f'}}\left( 2 \right) = \frac{1}{{48}}\). If \(\mathop \smallint \nolimits_6^{{\rm{f}}\left( {\rm{x}} \right)} \left( {4{{\rm{t}}^3}} \right){\rm{dt}} = \left( {{\rm{x}} - 2} \right){\rm{g}}\left( {\rm{x}} \right),{\rm{\;then\;}}\mathop {{\rm{lim}}}\limits_{{\rm{x}} \to 2} {\rm{\;g}}\left( {\rm{x}} \right)\)is equal to:1. 182. 243. 124. 36 |
Answer» Correct Answer - Option 1 : 18 The given equation is: \(\mathop \smallint \nolimits_6^{{\rm{f}}\left( {\rm{x}} \right)} \left( {4{{\rm{t}}^3}} \right){\rm{dt}} = \left( {{\rm{x}} - 2} \right){\rm{g}}\left( {\rm{x}} \right)\) Now, g(x), \(\Rightarrow {\rm{g}}\left( {\rm{x}} \right) = \frac{{\mathop \smallint \nolimits_6^{{\rm{f}}\left( {\rm{x}} \right)} \left( {4{{\rm{t}}^3}} \right){\rm{dt}}}}{{\left( {{\rm{x}} - 2} \right)}}\) On integrating, \(\Rightarrow {\rm{g}}\left( {\rm{x}} \right) = \frac{{\left[ {\frac{{4{{\rm{t}}^4}}}{4}} \right]_6^{{\rm{f}}\left( {\rm{x}} \right)}}}{{\left( {{\rm{x}} - 2} \right)}}\) \(\Rightarrow {\rm{g}}\left( {\rm{x}} \right) = \frac{{\left[ {{{\rm{t}}^4}} \right]_6^{{\rm{f}}\left( {\rm{x}} \right)}}}{{\left( {{\rm{x}} - 2} \right)}}\) \(\Rightarrow {\rm{g}}\left( {\rm{x}} \right) = \frac{{{{\left( {{\rm{f}}\left( {\rm{x}} \right)} \right)}^4} - {6^4}}}{{\left( {{\rm{x}} - 2} \right)}}\) Now applying the limit given in question, \(\Rightarrow \mathop {\lim }\limits_{{\rm{x}} \to 2} {\rm{g}}\left( {\rm{x}} \right) = \mathop {\lim }\limits_{{\rm{x}} \to 2} \frac{{{{\left( {{\rm{f}}\left( {\rm{x}} \right)} \right)}^4} - {6^4}}}{{\left( {{\rm{x}} - 2} \right)}}\) Which gives \(\left( {\frac{0}{0}} \right)\)form. On applying L'Hospital rule, \(\Rightarrow \mathop {\lim }\limits_{{\rm{x}} \to 2} {\rm{g}}\left( {\rm{x}} \right) = \mathop {\lim }\limits_{{\rm{x}} \to 2} \frac{{4{{\left( {{\rm{f}}\left( {\rm{x}} \right)} \right)}^3}.{\rm{f'}}\left( {\rm{x}} \right)}}{1}\) On direct substitution, ⇒ g(x) = 4(f(2))3.f'(2) ∵ f(2) = 6 (given) and \({\rm{f'}}\left( 2 \right) = \frac{1}{{48}}{\rm{}}\left( {{\rm{given}}} \right)\) \(\Rightarrow {\rm{g}}\left( {\rm{x}} \right) = 4 \times {\left( 6 \right)^3} \times \frac{1}{{48}}\) \(\Rightarrow {\rm{g}}\left( {\rm{x}} \right) = 216 \times \frac{1}{{12}}\) ∴ g(x) = 18 |
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