JCECE Engineering JCECE Engineering Solved Paper-2009

  • question_answer
    if the complex numbers \[\sin x+i\cos 2x\] and \[\cos x-i\sin 2x\] are complex conjugate to each other, then the value of \[x\] is

    A) \[\frac{\pi }{4}\]                                              

    B) \[\frac{\pi }{8}\]

    C) \[\frac{3\pi }{4}\]                                            

    D)  None of these

    Correct Answer: D

    Solution :

    Let          \[{{z}_{1}}=\sin x+i\cos 2x\] and        \[{{z}_{2}}=\cos x-i\sin 2x\] According to the given condition,                 \[{{\bar{z}}_{1}}={{z}_{2}}\] \[\Rightarrow \]               \[\sin x-i\cos 2x=\cos x-i\sin 2x\] \[\Rightarrow \]               \[\sin x=\cos x\]and\[\cos 2x=\sin 2x\] \[\Rightarrow \]               \[\tan x=1\]and\[\tan 2x=1\] \[\Rightarrow \]               \[x=\frac{\pi }{4},\,\,\frac{5\pi }{4},...\]and\[x=\frac{\pi }{8},\,\,\frac{5\pi }{8},\,\,...\] Hence, there exists no value of \[x\] common.


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