JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    \[f(x)\] and \[g(x)\] are two differentiable function on \[[0,\,2]\] such that \[f''(x)-g''(x)=0,f'(1)=2,g'(1)=4\], \[f(2)=3\], \[g(2)=9,\] then \[f(x)-g(x)\] at \[x=3/2\] is  [AIEEE 2002]

    A)            0

    B)            2

    C)            10

    D)            ? 5

    Correct Answer: D

    Solution :

               \[{}^{\left( x\frac{dy}{dx}-y \right)}/{}_{{{x}^{2}}=\frac{1}{x}-\frac{1}{a+bx}b=\frac{a}{x(a+bx)}}\]                    Integrate w.r.t. x, \[{f}'(x)-{g}'(x)=c\]            At \[x=1\], \[{f}'(1)-{g}'(1)=c\] Þ \[2-4=c\] Þ \[c=-2\]            Hence, \[{f}'(x)-{g}'(x)=-2\].            Again integrate w.r.t. x, \[f(x)-g(x)=-2x+{{c}_{1}}\].            At\[x=2\],\[f(2)-g(2)=-2\times 2+{{c}_{1}}\]Þ \[3-9+4={{c}_{1}}\]                                                           Þ \[{{c}_{1}}=-2\]            Then \[f(x)-g(x)=-\,2x-2=-\,(2x+2)\]                    \ \[f(3/2)-g(3/2)=-\,\left( 2\times \frac{3}{2}+2 \right)=-\,5\].


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