JEE Main & Advanced Mathematics Straight Line Question Bank Angle between two straight lines, Bisector of angle between two lines

  • question_answer
    The equation of the line which bisects the obtuse angle between the lines \[x-2y+4=0\] and \[4x-3y+2=0\], is  [IIT 1979]

    A)            \[(4-\sqrt{5})x-(3-2\sqrt{5})y+(2-4\sqrt{5})=0\]

    B)            \[(4+\sqrt{5})x-(3+2\sqrt{5})y+(2+4\sqrt{5})=0\]

    C)            \[(4+\sqrt{5})x+(3+2\sqrt{5})y+(2+4\sqrt{5})=0\]

    D) None of these

    Correct Answer: A

    Solution :

    The equations of the bisectors of the angles between the lines are \[\frac{x-2y+4}{\sqrt{1+4}}=\pm \frac{4x-3y+2}{\sqrt{16+9}}\] Taking positive sign, then                    \[(4-\sqrt{5})x-(3-2\sqrt{5})\text{ }y-(4\sqrt{5}-2)=0\]               .....(i) and negative sign gives                           \[(4+\sqrt{5})x-(2\sqrt{5}+3)y+(4\sqrt{5}+2)=0\]   .....(ii) Let \[\theta \]be the angle between the line (i) and one of the given line, then  \[\tan \theta =\left| \frac{\frac{1}{2}-\frac{4-\sqrt{5}}{3-2\sqrt{5}}}{1+\frac{1}{2}.\frac{4-\sqrt{5}}{3-2\sqrt{5}}} \right|=\sqrt{5}+2>1\] Hence the line (i) bisects the obtuse angle between the given lines.


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