Maths Olympiad Prep

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Combinatorics Difficulty 6.6 National olympiad Find the answer

Is it possible to split all straight lines in a plane into the pairs of perpendicular lines, so that every line belongs to a single pair?

Solution

1. Set up an x-y coordinate system: Consider a Cartesian coordinate system where each line can be represented by an equation in the form y=mx+c y = mx + c or x=k x = k .

2. Identify perpendicular lines: Two lines are perpendicular if the product of their slopes is 1-1. For example, if one line has a slope m m , the perpendicular line will have a slope 1m -\frac{1}{m} .

3. Consider lines parallel to the x- and y-axes: Lines parallel to the x-axis have the form y=c y = c (horizontal lines), and lines parallel to the y-axis have the form x=k x = k (vertical lines). These lines are always perpendicular to each other.

4. Pairing horizontal and vertical lines: For any given point (a,a)(a, a), the horizontal line through this point is y=a y = a and the vertical line through this point is x=a x = a . These two lines are perpendicular and can be paired together.

5. Generalization: The argument can be extended to any point (a,b)(a, b) in the plane. The horizontal line through this point is y=b y = b and the vertical line through this point is x=a x = a . These lines are also perpendicular and can be paired together.

6. Conclusion: Since every line parallel to the x-axis can be paired with a line parallel to the y-axis through some point (a,b)(a, b), it is possible to pair all such lines into perpendicular pairs.

However, this argument only covers lines parallel to the x- and y-axes. For lines with arbitrary slopes, the problem becomes more complex. Consider lines with slopes m1 m_1 and m2 m_2 such that m1m2=1 m_1 \cdot m_2 = -1 . There is no guarantee that every line can be paired with another line to satisfy this condition, as the number of lines with a specific slope is not necessarily equal to the number of lines with the negative reciprocal slope.

Therefore, it is not possible to pair all straight lines in a plane into pairs of perpendicular lines such that every line belongs to a single pair.

The final answer is False.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.