JEE Main & Advanced Sample Paper JEE Main Sample Paper-46

  • question_answer
    If the total number of m elements subsets of the set \[A=\{{{a}_{1}},\,{{a}_{2}},\,{{a}_{3}},...,{{a}_{n}}\}\] is \[\lambda \] times the number of 3 elements subsets containing \[{{a}_{4}},\] then \[n\] is

    A)  \[(m-1)\,\lambda \]                 

    B)  \[m\lambda \]

    C)  \[(m+1)\,\lambda \]               

    D)  \[0\]

    Correct Answer: B

    Solution :

     Total number of m elements subsets of \[A{{=}^{n}}{{C}_{m}}\]and number of m elements subsets of A each containing the element \[{{a}_{4}}{{=}^{n-1}}{{C}_{m-1}}\] According to question, \[^{n}{{C}_{m}}=\lambda \cdot {{\,}^{n-1}}{{C}_{m-1}}\] \[\Rightarrow \]            \[\frac{n}{m}\cdot {{\,}^{n-1}}{{C}_{m-1}}={{\lambda }^{n-1}}{{C}_{m-1}}\] \[\Rightarrow \]            \[\lambda =\frac{n}{m}\,\,\Rightarrow \,\,\,n=m\lambda \]


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