JEE Main & Advanced JEE Main Paper Phase-I (Held on 07-1-2020 Evening)

  • question_answer
    The number of ordered pairs (r, k) for which \[6\cdot {{\,}^{35}}{{C}_{r}}=({{k}^{2}}3)\,\cdot {{\,}^{36}}{{C}_{r}}_{+1},\]where k is an integer, is [JEE MAIN Held on 07-01-2020 Evening]

    A) 3                     

    B) 6

    C) 2                     

    D) 4

    Correct Answer: D

    Solution :

    [d] \[\because \,\,\,\,{{\,}^{36}}{{C}_{r+1}}\cdot ({{k}^{2}}3)\,={{\,}^{35}}{{C}_{r}}\times 6\] \[\frac{36!}{(r+1)!(35-r)!}\cdot ({{k}^{2}}-3)=\frac{35!}{r!(35-r)}\times 6\] \[6({{k}^{2}}-3)=r+1\] \[\therefore \,\,\,\,{{k}^{2}}=3+\frac{r+1}{6}\] \[\therefore \] r can be 5 and 35 When r = 5 then \[k=\pm \,2\]and when r = 35, then\[k=\pm \,3\]. \[\therefore \] Total number of ordered pairs = 4


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