CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2011

  • question_answer
    If\[\omega \]is an imaginary cube root of unity, then the value of \[(1-\omega +{{\omega }^{2}}).(1-{{\omega }^{2}}+{{\omega }^{4}}).(1-{{\omega }^{4}}+{{\omega }^{8}})....\](\[2n\]factors) is

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

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

    C)  1                                            

    D)  0

    Correct Answer: A

    Solution :

    Given,     \[{{\omega }^{3}}=1,1+\omega +{{\omega }^{2}}=0\]      ...(i) Now, \[(1-\omega +{{\omega }^{2}}).(1-{{\omega }^{2}}+{{\omega }^{4}}).(1-{{\omega }^{4}}+{{\omega }^{8}}).\] \[(1-{{\omega }^{8}}+{{\omega }^{16}})........2n\] factors                 \[=(1-\omega +{{\omega }^{2}})\,\,(1-{{\omega }^{2}}+\omega )\] \[.(1-\omega +{{\omega }^{2}}).(1-{{\omega }^{2}}+\omega )......2n\] factors [from Eq. (i)]                 \[=(-\omega -\omega ).(-{{\omega }^{2}}-{{\omega }^{2}}).(-\omega -\omega ).(-{{\omega }^{2}}-{{\omega }^{2}})\] ... 2n factors [from Eq. (i)]                 \[=(-2\omega ).(-2{{\omega }^{2}}).(-2\omega ).(-2{{\omega }^{2}})....2n\]factors                 \[={{(-2)}^{2n}}{{(\omega )}^{n}}{{({{\omega }^{2}})}^{n}}\]                 \[={{(-1)}^{2n}}{{(2)}^{2n}}{{\omega }^{3n}}\]                 \[={{2}^{n}}.{{({{\omega }^{3}})}^{n}}={{2}^{2n}}{{(1)}^{n}}={{2}^{2n}}\]   [from Eq. (i)]


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