JEE Main & Advanced Mathematics Differentiation Question Bank Mock Test - Limits and Derivatives

  • question_answer
    Let y be an implicit function of x defined by \[{{x}^{2x}}-2{{x}^{x}}\cot y-1=0\]. Then y'(1) equals

    A) -1                    

    B) 1

    C) \[log\text{ }2\]               

    D) \[-log\text{ }2\]

    Correct Answer: A

    Solution :

    [a] \[{{x}^{2x}}-2{{x}^{x}}\cot y-1=0\]      (i) Now at x=1, \[1-2\cot y-1=0\Rightarrow \cot y=0\Rightarrow y=\frac{\pi }{2}\] Now differentiating (i) w.r.t,x, we get \[2{{x}^{2x}}(1+logx)-2\left[ {{x}^{x}}(-cose{{c}^{2}}y)\frac{dy}{dx}+\cot y{{x}^{x}}(1+logx) \right]=0\] Now at \[(1,\pi /2)\], \[2(1+log1)-2\left[ 1(-1){{\left( \frac{dy}{dx} \right)}_{1,\pi /2}}+0 \right]=0\] \[\Rightarrow 2+2{{\left( \frac{dy}{dx} \right)}_{(1,\pi /2)}}=0\] \[\Rightarrow {{\left( \frac{dy}{dx} \right)}_{(1,\pi /2)}}=-1\]


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