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

  • question_answer
    Six points in a plane be joined in all possible ways by indefinite straight lines, and if no two of them be coincident or parallel, and no three pass through the same point (with the exception of the original 6 points). The number of distinct points of intersection is equal to

    A) 105

    B) 45

    C) 51

    D) None of these

    Correct Answer: C

    Solution :

    Number of lines from 6 points\[{{=}^{6}}{{C}_{2}}=15\]. Points of intersection obtained from these lines\[{{=}^{15}}{{C}_{2}}=105\]. Now we find the number of times, the original 6 points come. Consider one point say\[{{A}_{1}}\]. Joining \[{{A}_{1}}\]to remaining 5 points, we get 5 lines, and any two lines from these 5 lines give \[{{A}_{1}}\] as the point of intersection. \[\therefore \]\[{{A}_{1}}\] come\[^{5}{{C}_{2}}=10\] times in 105 points of intersections. Similar is the case with other five points. \[\therefore \] 6 original points come \[6\times 10=60\] times in points of intersection. Hence the number of distinct points of intersection\[=105-60+6=51\].


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