JEE Main & Advanced Mathematics Circle and System of Circles Question Bank System of circles

  • question_answer
    The equation of the circle having the lines  \[{{x}^{2}}+2xy+3x+6y=0\]as its normals and having size just sufficient to contain the circle \[x(x-4)+y(y-3)=0\]is  [Roorkee 1990]

    A)            \[{{x}^{2}}+{{y}^{2}}+3x-6y-40=0\]                            

    B)            \[{{x}^{2}}+{{y}^{2}}+6x-3y-45=0\]

    C)            \[{{x}^{2}}+{{y}^{2}}+8x+4y-20=0\]                          

    D)            \[{{x}^{2}}+{{y}^{2}}+4x+8y+20=0\]

    Correct Answer: B

    Solution :

               Given circle is \[\left( 2,\ \frac{3}{2} \right)\text{ },\ \frac{5}{2}={{r}_{1}}\] (say)                    Required normals of circlres are \[x+3=0,\ x+2y=0\]                    which intersect at the centre \[\left( -3,\ \frac{3}{2} \right)\text{ },\ {{r}_{2}}=\]radius (say).                    2nd circle just contains the 1st                    i.e., \[{{C}_{2}}{{C}_{1}}={{r}_{2}}-{{r}_{1}}\Rightarrow {{r}_{2}}=\frac{15}{2}\].


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