JEE Main & Advanced JEE Main Paper (Held on 09-4-2019 Afternoon)

  • question_answer
    If \[\int_{{}}^{{}}{{{e}^{\sec x}}}(sec\,x\,tan\,xf(x)+(sec\,x\,tan\,x+se{{c}^{2}}x)dx\]\[={{e}^{\sec \,x}}f(x)+C,\]then a possible choice of \[f\left( x \right)\] is :-                                                                                                  [JEE Main 9-4-2019 Afternoon]

    A) \[\sec x-\tan x-\frac{1}{2}\]    

    B) \[x\sec x+\tan x+\frac{1}{2}\]

    C) \[\sec x+x\tan x-\frac{1}{2}\]

    D)   \[\sec x+\tan x+\frac{1}{2}\]

    Correct Answer: D

    Solution :

    \[\int_{{}}^{{}}{{{e}^{\sec x}}}(sec\,x\,tan\,xf(x)+(sec\,x\,tan\,x+se{{c}^{2}}x)dx\] \[={{e}^{\sec \,x}}f(x)+C,\] Diff. both sides w.r.t. 'x' \[{{e}^{\sec \,x}}(sec\,x\,tan\,\text{x}f(x)+(sec\,x\,tan\,x+{{\sec }^{2}}x))\] \[={{e}^{\sec \,x}}.sec\,x\,tan\,x\,f(x)+{{e}^{\sec x}}f'(x)\] \[f'(x)=se{{c}^{2}}x+\tan x\sec x\] \[\Rightarrow \]\[f(x)=tan\,x+sec\,x+c\]


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