JEE Main & Advanced JEE Main Paper (Held On 11-Jan-2019 Evening)

  • question_answer
    Set \[{{S}_{n}}=1+q+{{q}^{2}}+...+{{q}^{n}}\]and \[{{T}_{n}}=1+\left( \frac{q+1}{2} \right)+{{\left( \frac{q+1}{2} \right)}^{2}}+...+{{\left( \frac{q+1}{2} \right)}^{n}}\]where q is a real number and \[q\ne 1.\] It \[^{101}{{C}_{1}}{{+}^{101}}{{C}_{2}}{{S}_{1}}+...{{+}^{101}}{{C}_{101}}{{S}_{100}}=\alpha {{T}_{100}},\]then \[\alpha \] is equal to [JEE  Main Online Paper (Held on 11-jan-2019 Evening)]

    A) \[{{2}^{100}}\]                                              

    B) \[{{2}^{99}}\]

    C) 202                              

    D)   200

    Correct Answer: A

    Solution :

    Here,\[^{101}{{C}_{1}}{{+}^{101}}{{C}_{2}}{{S}_{1}}+...{{+}^{101}}{{C}_{101}}{{S}_{100}}=\alpha {{T}_{100}}\] \[\Rightarrow \]\[^{101}{{C}_{1}}{{+}^{101}}{{C}_{2}}(1+q){{+}^{101}}{{C}_{3}}(1+q+{{q}^{2}})+\]\[...{{+}^{101}}{{C}_{101}}(1+q+...+{{q}^{100}})\] \[=2\alpha \frac{\left( 1-{{\left( \frac{1+q}{2} \right)}^{101}} \right)}{(1-q)}\] \[\Rightarrow \]\[^{101}{{C}_{1}}(1-q){{+}^{101}}{{C}_{2}}(1-{{q}^{2}})+...\]\[{{+}^{101}}{{C}_{101}}(1-{{q}^{101}})\] \[=2\alpha \left( 1-{{\left( \frac{1+q}{2} \right)}^{101}} \right)\] \[\Rightarrow \]\[({{2}^{101}}-1)-[{{(1+q)}^{101}}-1]\] \[=2\alpha \left( 1-{{\left( \frac{1+q}{2} \right)}^{101}} \right)\] \[\Rightarrow \]\[={{2}^{101}}\left( 1-{{\left( \frac{1+q}{2} \right)}^{101}} \right)=2\alpha \left( 1-{{\left( \frac{1+q}{2} \right)}^{101}} \right)\] \[\Rightarrow \]\[\alpha ={{2}^{100}}\]


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