CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2002

  • question_answer
    Let\[{{z}_{1}}\]and\[{{z}_{2}}\]be nth roots of unity which subtend a right angle at the origin. Then \[n\]must be of the form:

    A) \[4k+3\]                              

    B) \[4k\]

    C) \[4k+3\]                              

    D) \[4k+2\]

    Correct Answer: B

    Solution :

    \[z={{1}^{1/n}}={{(\cos 0+i\sin 0)}^{1/n}}\] \[=\cos \frac{2\pi \,r}{n}+i\sin \frac{2\pi r}{n}={{e}^{\frac{i\,2\pi r}{n}}}\] where r varies from 0 to \[n-1\]where each root is unimodular as \[|{{e}^{i\theta }}|=1.\] Let \[{{z}_{1}}=1,{{z}_{2}}={{e}^{i\frac{2\pi \,k}{n}}},\]where \[{{z}_{2}}-0=({{z}_{1}}-0){{e}^{\pi /x}}\] by given  condition or \[{{e}^{i\frac{2k\,\pi }{n}}}={{e}^{\frac{\pi }{2}}}\Rightarrow \]\[n=4k\]


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