JEE Main & Advanced Mathematics Circle and System of Circles Question Bank Chord of contact of tangent, Pole and Polar

  • question_answer
     The pole of the straight line \[9x+y-28=0\]with respect to circle \[2{{x}^{2}}+2{{y}^{2}}-3x+5y-7=0\], is [RPET 1990, 99; MNR 1984; UPSEAT  2000]

    A)            (3, 1)                                         

    B)            (1, 3)

    C)            (3, -1)                                       

    D)            (-3, 1)

    Correct Answer: C

    Solution :

               \[{{x}^{2}}+{{y}^{2}}-\frac{3}{2}x+\frac{5}{2}y-\frac{7}{2}=0\]                    \[\Rightarrow {{\left( x-\frac{3}{4} \right)}^{2}}+{{\left( y+\frac{5}{4} \right)}^{2}}-\frac{9}{16}-\frac{25}{16}-\frac{7}{2}=0\]                    \[\Rightarrow {{\left( x-\frac{3}{4} \right)}^{2}}+{{\left( y+\frac{5}{4} \right)}^{2}}-\frac{45}{8}=0\]                    Put \[X=x-\frac{3}{4}\] and \[Y=y+\frac{5}{4}\], we get the equation of circle \[{{X}^{2}}+{{Y}^{2}}-\frac{45}{8}=0\] and the line \[9X+Y-\frac{45}{2}=0\]                    Hence pole \[\equiv \left( \frac{9\times \frac{45}{8}}{\frac{45}{2}},\ \ \frac{1\times \frac{45}{8}}{\frac{45}{2}} \right)\equiv \left( \frac{9}{4},\ \frac{1}{4} \right)\],                    But \[x=\frac{9}{4}+\frac{3}{4}\] and \[y=\frac{1}{4}-\frac{5}{4}=-1\].                    Hence the pole is (3, -1).


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