J & K CET Engineering J and K - CET Engineering Solved Paper-2014

  • question_answer
    Find \[^{n}{{C}_{21}}\] if \[^{n}{{C}_{10}}{{=}^{n}}{{C}_{11}}.\]

    A)  \[1\]                  

    B)  \[0\]

    C)  \[11\]                 

    D)  \[10\]

    Correct Answer: A

    Solution :

    Given, \[^{n}{{C}_{10}}{{=}^{n}}{{C}_{11}}\] \[\Rightarrow \] \[\frac{n!}{10!(n-10)!}=\frac{n!}{11!(n-11)!}\] \[\Rightarrow \] \[11\times 10!(n-11)!=0!(n-10)(n-11)!\] \[\Rightarrow \] \[11=n-10\] \[\Rightarrow \] \[n=11+10=21\] Now, \[^{n}{{C}_{21}}{{=}^{21}}{{C}_{21}}=1\]


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