JEE Main & Advanced JEE Main Paper (Held on 08-4-2019 Afternoon)

  • question_answer
    If the fourth term in the binomial expansion of\[{{\left( \sqrt{\frac{1}{{{x}^{1+{{\log }_{10}}x}}}}+{{x}^{\frac{1}{12}}} \right)}^{6}}\]is equal to 200, and\[x>1,\] then the value of x is : [JEE Main 8-4-2019 Afternoon]

    A) \[{{10}^{3}}\]                     

    B) 100

    C) \[{{10}^{4}}\]         

    D)   10

    Correct Answer: D

    Solution :

    \[200={}^{6}{{C}_{3}}{{\left( \frac{1}{{{X}^{x+{{\log }_{10}}x}}} \right)}^{\frac{3}{2}}}\times {{x}^{\frac{1}{4}}}\] \[\Rightarrow \]\[10={{x}^{\frac{3}{2(1+lo{{g}_{10}}x)}+\frac{1}{4}}}\] \[\Rightarrow \]\[1=\left( \frac{3}{2(1+t)}+\frac{1}{4} \right)t\] where \[t={{\log }_{10}}\text{x}\] \[\Rightarrow \]\[{{t}^{2}}+3t-4=0\] \[\Rightarrow \]\[t=1,-4\] \[\Rightarrow \]\[x=10,{{10}^{-4}}\] \[\Rightarrow \]\[x=10\,\,\,(As\,x>1)\]


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