JEE Main & Advanced Mathematics Binomial Theorem and Mathematical Induction Question Bank Critical Thinking

  • question_answer
    In the expansion of \[{{(x+a)}^{n}}\], the sum of odd terms is P and sum of even terms is Q, then the value of \[({{P}^{2}}-{{Q}^{2}})\] will be       [RPET 1997; Pb. CET 1998]

    A) \[{{({{x}^{2}}+{{a}^{2}})}^{n}}\]

    B) \[{{({{x}^{2}}-{{a}^{2}})}^{n}}\]

    C) \[{{(x-a)}^{2n}}\]

    D) \[{{(x+a)}^{2n}}\]

    Correct Answer: B

    Solution :

    \[{{(x+a)}^{n}}={{x}^{n}}+{{\,}^{n}}{{C}_{1}}{{x}^{n-1}}a{{+}^{{}}}\].....    \[=({{x}^{n}}+{{\,}^{n}}{{C}_{2}}{{x}^{n-2}}{{a}^{2}}+.......\]+\[{{(}^{n}}{{C}_{1}}{{x}^{n-1}}a+{{\,}^{n}}{{C}_{3}}{{x}^{n-3}}{{a}^{3}}+.....)\] =\[P+Q\] \[\therefore \] \[{{(x-a)}^{n}}=P-Q,\] As the terms are alter. +ve and -ve \[\therefore \] \[{{P}^{2}}-{{Q}^{2}}=(P+Q)(P-Q)={{(x+a)}^{n}}{{(x-a)}^{n}}\]    \[{{P}^{2}}-{{Q}^{2}}={{({{x}^{2}}-{{a}^{2}})}^{n}}\]


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