CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2007

  • question_answer
    The number of common tangents to the circles  \[{{x}^{2}}+{{y}^{2}}=4\]and \[{{x}^{2}}+{{y}^{2}}-6x-8y-24=0\]is,

    A)  \[3\]                                    

    B)  \[4\]

    C)  \[2\]                                    

    D)  \[1\]

    Correct Answer: B

    Solution :

    Given, equation of circles are \[{{x}^{2}}+\text{ }{{y}^{2}}=4,\]whose radius \[=2,\] centre \[=(0,0)\] and \[{{x}^{2}}+{{y}^{2}}-6x-8y-24=0,\] whose radius \[=\sqrt{9+16-24}=\sqrt{1}=1\] and centre \[=(3,4)\] Now,  \[{{c}_{1}}\,{{c}_{2}}=\sqrt{{{(3-0)}^{2}}+{{(4-0)}^{2}}}\]                                 \[=\sqrt{9+16}=5\]                 \[{{a}_{1}}+{{a}_{2}}=2+1=3\] Since,    \[{{c}_{1}}{{c}_{2}}>{{a}_{1}}+{{a}_{2}}\] Number of common tangents \[=4\]


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