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JEE Main & Advanced Sample Paper JEE Main Sample Paper-4

  • question_answer
    The coefficients of 3 consecutive terms in the expansion of \[{{(1+x)}^{n}}\] are in ratio 1: 7 :35 then value of n is

    A)  35                                         

    B)  46

    C)  23                                         

    D)  \[4k+1\](where\[K\in {{l}^{+}}\]

    Correct Answer: C

    Solution :

    \[^{n}{{C}_{r}}{{:}^{n}}{{C}_{4+1}}{{:}^{n}}{{C}_{r+2}}=1:7:35\] \[\Rightarrow \]\[\frac{r+1}{n-r}=\frac{1}{7}\Rightarrow 8r=n-7\] \[\And \frac{r+2}{n-(r+1)}=\frac{1}{5}\Rightarrow 6r=n-11\] Solving above equations n = 23


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