JEE Main & Advanced Mathematics Permutations and Combinations Question Bank Geometrical Problems

  • question_answer
    There are n points in a plane of which p points are collinear. How many lines can be formed from these points [Karnataka CET  2002]

    A) \[^{(n-p)}{{C}_{2}}\]

    B) \[^{n}{{C}_{2}}-{{\,}^{p}}{{C}_{2}}\]

    C)   \[^{n}{{C}_{2}}-{{\,}^{p}}{{C}_{2}}+1\]

    D) \[^{n}{{C}_{2}}-{{\,}^{p}}{{C}_{2}}-1\]

    Correct Answer: C

    Solution :

    Given, total number of points = n and number of collinear points = p.  We know that one line has two end points.  Therefore total number of lines =\[^{n}{{C}_{2}}\].  Since p points are collinear, therefore total number of lines drawn from collinear points =\[^{\,p}{{C}_{2}}\].  We also know that, corresponding to the line of collinearity, one will also be added. Therefore number of lines = \[^{n}{{C}_{2}}\,-{{\,}^{p}}{{C}_{2}}+1.\]


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