JEE Main & Advanced Mathematics Pair of Straight Lines Question Bank Angle between the pair of straight lines, Condition for parallel and perpendicular lines

  • question_answer
    The acute angle formed between the lines joining the origin to the points of intersection of the curves \[{{x}^{2}}+{{y}^{2}}-2x-1=0\] and \[x+y=1\], is [MP PET 1998]

    A)            \[{{\tan }^{-1}}\left( -\frac{1}{2} \right)\]                    

    B)            \[{{\tan }^{-1}}2\]

    C)            \[{{\tan }^{-1}}\frac{1}{2}\]   

    D)            \[{{60}^{o}}\]

    Correct Answer: B

    Solution :

               From \[x+y=1,\]to make the curve \[{{x}^{2}}+{{y}^{2}}-2x-1=0\] homogenous.            \[\Rightarrow {{x}^{2}}+{{y}^{2}}-2x(x+y)-{{(x+y)}^{2}}=0\]            \[\therefore 2{{x}^{2}}+4xy=0\] or \[{{x}^{2}}+2xy=0\]            \[\therefore \tan \theta =\frac{2\sqrt{{{h}^{2}}-ab}}{a+b}\] and \[a=1,\ b=0,\ h=1\]            \[\therefore \tan \theta =\frac{2\sqrt{{{1}^{2}}-0}}{1}\Rightarrow \theta ={{\tan }^{-1}}(2)\].


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