JEE Main & Advanced Sample Paper JEE Main - Mock Test - 33

  • question_answer
    If \[f(\alpha )=x\,cos\,\alpha +y\,\,\sin \alpha -p(\alpha )\], then the lines \[f(\alpha )=0\] and \[f(\beta )=0\] are \[\bot \] to each other, if

    A) \[\alpha =\beta \]           

    B)        \[\alpha +\beta =\pi /2\]

    C) \[\alpha -\beta =\pi /2\]  

    D)        \[\alpha \pm \beta =\pi /2\]

    Correct Answer: C

    Solution :

    Given that \[f(\alpha )=x\,cos\,\alpha +y\,\,\sin \,\,\alpha -p(\alpha )\] and \[f(\beta ) =x\,cos \beta  +y\,sin \beta -p(\beta )\] \[\because \] both lines are perpendicular to each other. \[\therefore \,\,{{a}_{1}}{{a}_{2}}+{{b}_{1}}{{b}_{2}}=0\,\,\Rightarrow \,\,\cos \,\alpha \,cos\,\beta +\sin \alpha \sin \beta =0\]\[\Rightarrow \,\,\,\cos (\alpha -\beta )=0\] \[\Rightarrow \,\,\,(\alpha -\beta )=\frac{\pi }{2}\]


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