KVPY Sample Paper KVPY Stream-SX Model Paper-8

  • question_answer
    Let \[f(x)={{a}^{x}}(a>0)\] be written as \[f(x)=g(x)+h(x),\] where \[g(x)\] is an even function and \[h(x)\] is an odd function. Then the value of the \[g(x+y)+g(x-y)\] is -

    A) \[2g(x).g(y)\]                  

    B) \[2g(x+y).g(x-y)\]

    C) \[2g(x)\]            

    D) none of these

    Correct Answer: A

    Solution :

    \[f(x)=\underset{even}{\mathop{g(x)}}\,+\underset{odd}{\mathop{h(x)}}\,\]
    \[\Rightarrow g(x)\,\,=\,\,\frac{{{a}^{x}}+{{a}^{-x}}}{2}\]
    \[g\,\,(x+y)+g\,\,(x-y)\]
    \[=\frac{{{a}^{x+y}}+{{a}^{-y+x}}}{2}+\frac{{{a}^{x-y}}+{{a}^{y-x}}}{2}\]
    \[=\frac{1}{2}[{{a}^{x}}.{{a}^{y}}+{{a}^{x}}.{{a}^{-y}}+{{a}^{y}}.{{a}^{-x}}+{{a}^{-x}}{{a}^{-y}}]\]
    \[=\frac{1}{2}[{{a}^{x}}({{a}^{y}}+{{a}^{-y}})+{{a}^{-x}}({{a}^{y}}+{{a}^{-y}})]\]
    \[=\frac{1}{2}({{a}^{y}}+{{a}^{-y}})({{a}^{x}}+{{a}^{-x}})\]
    \[=2g(x).\,\,g(y)\]


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