JEE Main & Advanced JEE Main Paper Phase-I (Held on 09-1-2020 Evening)

  • question_answer
    In the expansion of \[{{\left( \frac{x}{\cos \theta }+\frac{1}{x\sin \theta } \right)}^{16}},\] if \[{{l}_{1}}\] is the least value of the term independent of x when \[\frac{\pi }{8}\le \theta \le \frac{\pi }{4}\] and \[{{l}_{2}}\] is the least value of the term independent of x when \[\frac{\pi }{16}\le \theta \le \frac{\pi }{8},\] then the ratio \[{{l}_{2}}:{{l}_{1}}\] is equal to  [JEE MAIN Held on 09-01-2020 Evening]

    A) \[1:16\]

    B) \[16:1\]

    C) \[1:8\]

    D) \[8:1\]

    Correct Answer: B

    Solution :

    \[{{T}_{r+1}}={}^{16}{{C}_{r}}\cdot {{\left( \frac{x}{\cos \theta } \right)}^{16-r}}\cdot {{\left( \frac{1}{x\sin \theta } \right)}^{r}}\] \[={}^{16}{{C}_{r}}\cdot \frac{{{x}^{16-2r}}}{{{(\cos \theta )}^{16-r}}\cdot {{(\sin \theta )}^{r}}}\] \[\because \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,16-2r=0\Rightarrow r=8\] \[{{T}_{9}}=\frac{^{16}{{C}_{8}}\cdot {{2}^{8}}}{{{\sin }^{8}}2\theta }\] \[\because \,\,\,\,\,\,\,\,\,\,\,\sin 2\theta \] is increasing in \[\left[ \frac{\pi }{16},\frac{\pi }{4} \right]\] Hence,  \[{{l}_{1}}=\frac{^{16}{{C}_{8}}\cdot {{2}^{8}}}{{{\sin }^{8}}\left( \frac{\pi }{2} \right)}\]  and \[{{l}_{2}}=\frac{^{16}{{C}_{8}}\cdot {{2}^{8}}}{{{\sin }^{8}}\left( \frac{\pi }{4} \right)}\] \[\frac{{{l}_{2}}}{{{l}_{1}}}=\frac{{{\sin }^{8}}\left( \frac{\pi }{2} \right)}{{{\sin }^{8}}\left( \frac{\pi }{4} \right)}=\frac{16}{1}\]


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