A question in Permutations and combinations

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Discussion Overview

The discussion revolves around determining the maximum number of intersection points possible between four circles and four straight lines. It explores the combinatorial aspects of intersections among these geometric figures, including both theoretical calculations and practical arrangements.

Discussion Character

  • Mathematical reasoning
  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant asks for clarification on whether the question pertains to the maximum number of points where all figures intersect or just at least two intersect.
  • Another participant calculates a total of 50 maximum intersection points based on the intersections between circles and lines, detailing the contributions from circle-circle, line-circle, and line-line intersections.
  • A subsequent reply agrees with the upper bound of 50 intersections but questions the feasibility of achieving all these intersections simultaneously, suggesting a construction method involving circles and lines to illustrate the point.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the calculated maximum of 50 intersections is achievable in practice, indicating a disagreement on the feasibility of the proposed arrangements.

Contextual Notes

The discussion includes assumptions about the arrangement of circles and lines, such as the general position of lines and the size of circles relative to one another, which may affect the validity of the intersection counts.

Vishalrox
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there are 4 circles and 4 straight lines...find the maximum number of intersecting points possible in the intersection of all these given figures...i can't get how to solve it...
 
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Do you mean the maximum number of points where they all intersect? Or just at least 2 intersect?
 
A circle can intersect another circle only twice so you would have:
6 intersects + 4 intersects + 2 intersects= 12

A straight line can intersect a circle a maximum of twice and there are four circles so:
8 intersects per line x 4 lines = 32 intersects.

Each line can also intersect the other lines at a single point so they overlap each of the four lines so not recounting an intersect would be:
3+2+1=6

Add them up:
12+32+6= 50 max intersects

Someone should check this ;)
 
mesa said:
A circle can intersect another circle only twice so you would have:
6 intersects + 4 intersects + 2 intersects= 12

A straight line can intersect a circle a maximum of twice and there are four circles so:
8 intersects per line x 4 lines = 32 intersects.

Each line can also intersect the other lines at a single point so they overlap each of the four lines so not recounting an intersect would be:
3+2+1=6

Add them up:
12+32+6= 50 max intersects
That's certainly an upper bound, but it's not immediately obvious that all these intersections are achievable simultaneously.
Start with some circle C. (This is not one of THE circles, it's just used for construction.)
You can arrange N equal circles, larger than C, such that each surrounds C. Clearly each pair of these intersects.
Any M straight lines in general position must intersect each other.
The region in which the intersections of the lines occurs can be bounded by a circle, D. Shrink that picture as necessary and fit D inside C. Necessarily, every line intersects every circle.
So any number of circles and lines can be arranged to achieve the upper bound.
 

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