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Question

What is the maximum number of possible points of intersection of four straight lines and a circle (intersection is between lines as well as circle and lines)?

This question was previously asked in
NDA 2 2024 GAT Question Paper (01-Sep-2024)
The correct answer is
14

Understanding Intersection Points: Lines and Circle

The problem asks us to find the absolute maximum number of distinct points where four straight lines and one circle can cross each other. This total count includes points where lines cross lines, and points where lines cross the circle.

Calculating Line-Line Intersections

To get the maximum number of intersections between the four lines themselves, we assume each line crosses every other line at a unique point (no parallel lines, no three lines meeting at one spot).

The formula for the maximum number of intersection points for n distinct lines is given by the combination formula \( \binom{n}{2} \), which calculates how many pairs of lines can be formed.

\( \text{Number of line-line intersections} = \binom{n}{2} = \frac{n(n-1)}{2} \)

With 4 lines (\(n=4\)), we calculate:

\( \binom{4}{2} = \frac{4 \times (4-1)}{2} = \frac{4 \times 3}{2} = \frac{12}{2} = 6 \)

So, the four lines can create a maximum of 6 intersection points among themselves.

Calculating Line-Circle Intersections

Now, let's consider the intersections between the lines and the circle. A single straight line can intersect a circle at most at two distinct points. To achieve the maximum total intersections, we assume each of the four lines intersects the circle at two separate points.

Calculation:

\( \text{Number of line-circle intersections} = (\text{Number of lines}) \times (\text{Max intersections per line with circle}) \)

\( \text{Number of line-circle intersections} = 4 \times 2 = 8 \)

Thus, the four lines create a maximum of 8 intersection points with the circle.

Total Maximum Intersection Points

To find the overall maximum number of intersection points, we sum the maximum points from the line-line intersections and the line-circle intersections.

\( \text{Total Maximum Intersections} = (\text{Max Line-Line Intersections}) + (\text{Max Line-Circle Intersections}) \)

\( \text{Total Maximum Intersections} = 6 + 8 = 14 \)

Therefore, the maximum possible number of intersection points for four straight lines and a circle is 14.

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