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What is the maximum number of points of intersection that can be achieved by arranging 8 straight lines and 4 circles in the plane?


The answer is 104.

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3 solutions

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Pranjal Jain
Dec 11, 2014

Case-1: Two straight lines intersect

Since two straight lines can intersect at only one point, maximum possible number of such points = ( 8 2 ) = 56 =\binom{8}{2}=56

Case-2: Two circles intersect

Since two circles can intersect at maximum of 2 points, maximum possible number of such points = 2 × ( 4 2 ) = 12 =2×\binom{4}{2}=12

Case-3: One circle and one line intersect

Since one circle and one line can intersect at maximum of two points, maximum possible number of such points = 2 × ( 8 1 ) ( 4 1 ) = 64 =2×\binom{8}{1}\binom{4}{1}=64

Adding up, we get 64 + 12 + 28 = 104 64+12+28=\boxed{104}

Perfect solution...Great. keep it up.

Sandeep Bhardwaj - 6 years, 6 months ago

104 104 is certainly an upper bound on the number of intersections, but is it also achievable? Can you show that there exists such a configuration?

Tijmen Veltman - 6 years, 5 months ago

It is achievable! But I am quite busy with studies now-a-days! Can you add a figure? @Sandeep Bhardwaj

Pranjal Jain - 6 years, 5 months ago
Anna Anant
Dec 24, 2014

Case-1: Two straight lines intersect Since two straight lines can intersect at only one point, maximum possiblenumber of such points Case-2: Two circles intersect Since two circles can intersect at maximum of 2 points, maximum possiblenumber of such points Case-3: One circle and one line intersect Since one circle and one line can intersect at maximum of two points, maximum possible number of such points Adding up, we get 64+12+28 = 104

Jyotsna Sharma
Dec 12, 2014

Max no of points by the intercestion of n circles and m straight lines is nP2+mC2+2nm nP2 gives the no of intersections of circles mP2 gibes no of intersection of straight lines 2mn gives the intersection of lines with circle since each straight line cuts a circle at two points Hence , 4P2+8P2+2 8 4 gives us the max intersections I.e., 12+28+64=104

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