How Lines Divide a Plane – Geometry Concept for CAT/XAT

How Lines Divide a Plane – A Fundamental Geometry Concept

1. Non-Overlapping Line Segments on a Plane

Let’s begin with understanding how a plane can be divided using n non-overlapping straight lines.

(a) Minimum Number of Regions Formed

If the segments do not intersect at all, then the plane will be divided into just \((n+1)\) regions.

(b) Maximum Number of Regions Formed

When each new line intersects all the previous lines (no two are parallel and no three are concurrent), the maximum number of regions the plane can be divided into is:

R(n) = \(\frac{n(n+1)}{2} + 1\)

(c) Maximum Number of Open Segments Created

Each new line intersects all existing lines at unique points, which creates new open segments. The maximum number of open segments is:

\(\text{Maximum open segments} = 2n\)

2. When Sets of Parallel Lines Are Involved

Now consider a scenario where we have:

\(n\) non-parallel and non-overlapping lines
\(m\) lines that are all parallel to each other
\(p\) lines that are also parallel to each other

We assume that the parallel sets (m and p) are not parallel to the original set of n lines or to each other.

Maximum Number of Segments Formed

Under these conditions, the maximum number of segments into which the plane could be divided is:

\(\frac{n(n+1)}{2} + 1 + m(n+1) + p(n+m+1)\)

Key Insight: This type of problem is frequently asked in exams like CAT, XAT, and SSC. Focus on visualizing line intersections and incrementally adding lines to maximize or minimize region formation.

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