KVPY Sample Paper KVPY Stream-SX Model Paper-15

  • question_answer
    Number of irrational terms in expansion of \[{{(\sqrt{3}+\sqrt{7})}^{17}}\] is equal to \[3k\text{ }+\text{ }{{k}^{2}}\] then \[k\]equals

    A) 7

    B) 3    

    C) 5

    D) None

    Correct Answer: B

    Solution :

    \[\therefore {{(\sqrt{3}+\sqrt{7})}^{17}}=\sum\limits_{r=0}^{17}{^{17}{{C}_{r}}={{(\sqrt{3})}^{r}}\,{{(\sqrt{7})}^{17-r}}}\]
        \[=\sum\limits_{r=0}^{17}{^{17}}{{C}_{r}}{{3}^{\frac{r}{2}}}\,\,{{7}^{\frac{17-r}{2}}}\]
    \[\because \]Both \[\frac{r}{2}\] & \[\frac{17-r}{2}\] can?t be integer at same so all term are irrational
    \[\therefore \]Total irrational terms are 18
    \[\therefore k=3\]


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