JEE Main & Advanced Sample Paper JEE Main - Mock Test - 8

  • question_answer
    If \[x+iy=\frac{3}{2+\cos \theta +i\sin \theta }\], then\[{{x}^{2}}+{{y}^{2}}\]is equal to

    A) \[3x-4\] 

    B) \[4x-3\]

    C) \[4x+3\]           

    D)  None of these

    Correct Answer: B

    Solution :

    [b]: Consider,\[x+iy=\frac{3}{2+\cos \theta +isin\theta }\] \[=\frac{3(2+cos\theta -i\,sin\theta )}{{{(2+\cos \theta )}^{2}}+si{{n}^{2}}\theta }\] \[=\frac{6+3cos\theta -3i\,sin\theta }{4+{{\cos }^{2}}\theta +4\cos \theta +{{\sin }^{2}}\theta }\] \[=\left[ \frac{6+3cos\theta }{5+4\cos \theta } \right]+i\left[ \frac{-3\sin \theta }{5+4\cos \theta } \right]\] \[\Rightarrow \]\[x=\frac{3(2+cos\theta )}{5+4\cos \theta },y=\frac{-3\sin \theta }{5+4\cos \theta }\] \[\therefore \]\[{{x}^{2}}+{{y}^{2}}=\frac{9[4+co{{s}^{2}}+4cos+si{{n}^{2}}]}{{{(5+4cos\theta )}^{2}}}\] \[=\frac{9}{5+4cos\theta }=4\left[ \frac{6+3\cos \theta }{5+4\cos \theta } \right]-3=4x-3\]


You need to login to perform this action.
You will be redirected in 3 sec spinner