11th Class Mathematics Sample Paper Mathematics Olympiad - Sample Paper-7

  • question_answer
    \[\underset{x\to 0}{\mathop{lim}}\,{{\log }_{\tan x}}(sinx)\]equal to:

    A)  0                                

    B)  \[-1\]             

    C)  ¥                                

    D)  1

    Correct Answer: D

    Solution :

    [d] \[\because y=\underset{x\to {{0}^{+}}}{\mathop{\lim }}\,{{\log }_{\tan x}}(\sin x)\] \[=\underset{x\to 0}{\mathop{\lim }}\,\frac{{{\log }_{e}}\sin x}{{{\log }_{e}}\tan x}\]      which is \[\frac{0}{0}\]form \[y=\underset{x\to {{0}^{+}}}{\mathop{\lim }}\,\frac{\frac{1}{\sin }.\cos x}{\frac{1}{\tan x}.{{\sec }^{2}}x}\]     [by L Hospital Rule] \[y=\underset{x\to {{0}^{+}}}{\mathop{\lim }}\,\frac{\frac{\cos x}{\sin x}}{\frac{{{\sec }^{2}}x}{\sin x}\times \cos x}=\underset{x\to 0}{\mathop{\lim }}\,\frac{\cos x}{\sec x}\] \[=\underset{x\to 0}{\mathop{\lim }}\,{{\cos }^{2}}x=1\]


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