Maximum points of intersection!

Find the maximum number of points of intersection of 5 circles and 8 straight lines.


The answer is 128.

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2 solutions

Puneet Sharma
Oct 17, 2015

If you select any two circles, you can have max 2 points of intersection. =(5c2)(2). If u select any two straight lines then you have max one point of intersection =( 8c2)(1). And if u select one circle and one straight line you get max 2 points of intersection =(8c1)(5c1)(2). hence total points add up to 20 +28 + 80= 128

Seth Christman
Oct 6, 2016

This is the same reasoning as Puneet Sharma's answer, just using LaTeX to make it visually clearer

2 2 circles intersect each other at most 2 2 times. The number of circle intersections is equal to 2 2 times the number of circle pairs, 2 × ( 5 2 ) = 2 × 10 = 20 2\times\binom{5}{2}=2\times10=20

2 2 lines intersect each other at most 1 1 time. The number of line intersections is equal to 1 1 times the number of line pairs, 1 × ( 8 2 ) = 1 × 28 = 28 1\times\binom{8}{2}=1\times28=28

Lastly, a line and a circle intersect at most 2 2 times. The number of line and circle intersections is equal to 2 2 times the number of line and circle pairs, 2 × 5 × 8 = 80 2\times5\times8=80

20 + 28 + 80 = 128 20+28+80=128

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