Enrol to start learning
Reading is open to everyone. Enrolling is free, and it is what unlocks the audio lessons, practice tests and progress tracking.
17.5. Question 8
Learn content
Interactive Audio Lesson
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Today, we will learn about distinct points in a two-dimensional space. Can anyone tell me what distinct points mean?
Does it mean the points cannot be at the same location?
Exactly! Each point has unique coordinates. Now, if we have distinct points in 2D, what kind of coordinates do we usually have?
We usually have both x and y coordinates.
Right! And for this problem, we will focus on integer coordinates. Next, let’s move to how we find midpoints.
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Who can tell me how to calculate the midpoint of two points, say P1 and P2?
I think we add the x coordinates and the y coordinates and divide by two.
Excellent! The formula is M = ((x1 + x2) / 2, (y1 + y2) / 2). Based on this formula, what can we infer if both x1 and x2 are odd? How about if both are even?
If both are odd, their sum would be even, so the midpoint will also be an integer!
Yes! This is key to our problem. Now, let's consider the pigeonhole principle.
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
Now, let's use the pigeonhole principle. Can anyone explain how we relate this to our points?
We have five points and only four combinations of odd/even x and y coordinates!
Exactly! By mapping the points based on their x and y coordinates—odd or even—we see that at least two points must fall into the same category. What does that mean for their midpoints?
It means those two points will have integer midpoints!
Perfectly understood! Let’s summarize this principle.
Unlock the classroom podcast
The transcript is free to read. A free account plays the conversation back.
In conclusion, we've proven that in any set of five distinct points, we can always find at least one pair whose midpoint is an integer. Can anyone summarize how we arrived at this conclusion?
By using the pigeonhole principle, we showed that at least two points share the same odd/even characteristic!
And because their sums are even, their midpoint will definitely be an integer.
Exactly! Great teamwork today!
Overview
Short Summary
This section discusses the application of the pigeonhole principle to prove the existence of a pair of distinct points in a two-dimensional plane with integer coordinates whose midpoint also has integer coordinates.
Medium Summary
In this section, the author explains how to prove that out of five distinct points in a two-dimensional space defined by integer coordinates, there always exists at least one pair of points such that their midpoint is an integer. This is demonstrated through the pigeonhole principle, where the nature of the x and y coordinates (odd/even) is used to establish a mapping.
Detailed Summary
Detailed Summary
In this section, we explore question number 8 from Tutorial 6, where the task is to show that among five distinct points in a two-dimensional plane with integer coordinates, there exists at least one pair whose midpoint also has integer coordinates. The concept revolves around the Pigeonhole Principle, which states that if there are more items than containers, at least one container must contain more than one item.
First, we denote our five distinct points as
- P1 (x1, y1),
- P2 (x2, y2),
- P3 (x3, y3),
- P4 (x4, y4),
- P5 (x5, y5).
For each point, we categorize the x and y coordinates based on whether they are even or odd. The possible combinations yield four groups: (even, even), (even, odd), (odd, even), and (odd, odd). Using the pigeonhole principle, since we are mapping five points to four combinations, at least two of these points must fall into the same group.
Let’s say points P1 and P2 map to the same group, which means their x-coordinates share the same odd/even status, and their y-coordinates share the same. Regardless of their specific values, the midpoint for these two points will also have integer coordinates. The formula for calculating the midpoint M of two points (x1, y1) and (x2, y2) is:
M = ((x1 + x2) / 2, (y1 + y2) / 2).
Since both x1 and x2 are either even or odd, their sum (x1 + x2), as well as (y1 + y2), will always be even. Thus, the midpoints will also be integers confirming that the statement holds true.
Reference YouTube Videos
Audio Book
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountYou are given here arbitrary distinct points in 2 dimensional planes. Each point will have an x-coordinate, y-coordinate and the points are having integer coordinates. So they are arbitrary points except that they are distinct.
Detailed Explanation
The problem introduces a scenario where we need to analyze five distinct points in a 2D plane, each with integer coordinates. The focus is on how these points can be paired to find midpoints with integer coordinates. This sets up the challenge of demonstrating that no matter how these points are selected, at least one pair will yield a midpoint with integer coordinates.
Examples & Analogies
Imagine you have five different locations on a city map marked with pins. Each pin represents a location with specific coordinates. The goal is to find two pins such that the point exactly halfway between them also lies on the grid of the map, meaning it has coordinates with whole numbers.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountSo just to recap if you have 2 points, a point with coordinates (x1, y1) and another point with coordinates (x2, y2), then the midpoint of the line joining these 2 points is given by the formula ((x1 + x2)/2, (y1 + y2)/2).
Detailed Explanation
The midpoint between two points, (x1, y1) and (x2, y2), is calculated using the formula that averages the x-coordinates and the y-coordinates of the two points. The resulting midpoint has coordinates that will only be integers if both x-coordinates are either both even or both odd, and the same applies to the y-coordinates.
Examples & Analogies
Think of a seesaw in a playground where you need to find the balance point. If both children are the same weight (representing the same parity - both even or both odd), the seesaw balances at a central point, which is akin to finding a midpoint between two coordinates.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountWe are trying to map this point depending upon what is the nature of their x-coordinate and y-coordinate. Depending upon whether the x-coordinate is even or odd, or whether the y-coordinate is odd or even, I have 4 possible combinations.
Detailed Explanation
The application of the pigeonhole principle involves categorizing our five points based on the parity (odd or even) of their x and y coordinates. There are four unique combinations: (odd, odd), (odd, even), (even, odd), (even, even). Since we have five points to choose from and only four combinations, by the pigeonhole principle, at least two points must fall into the same category.
Examples & Analogies
Imagine trying to place five different colored balls into four differently colored buckets. Since there are more balls than buckets, at least one bucket will hold at least two balls. Similarly, in our coordinate scenario, at least two points must share the same parity.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountOut of those 5 points… assume that out of those 2 points which are guaranteed to be mapped to the same ordered pair are the first 2 points. So say (x1, y1) and (x2, y2) are the 2 points… the midpoint of the line joining these 2 points will be ((x1 + x2)/2, (y1 + y2)/2).
Detailed Explanation
Once we identify two points that belong to the same parity category, we can calculate the midpoint using the midpoint formula. Since both pairs of x-coordinates are either both odd or both even, their sum will always be divisible by 2, ensuring the midpoint's x-coordinate is an integer. The same logic applies to the y-coordinates.
Examples & Analogies
If two friends walk the same distance from two different starting points but maintain the same pace, the midpoint of their journey will be at a concrete location on the path. This illustrates how we find a definite point (the midpoint) that can be determined precisely.
Unlock the audio lesson
The script is above and free to read. A free account plays it back, in the voice you pick.
Create a free accountAnd again in this case it is easy to see that y1 + y2 will be divisible by 2. And that shows that this statement is a correct statement.
Detailed Explanation
The conclusion emphasizes the correctness of our initial statement, proving that among the five points chosen, there exists at least one pair of points whose midpoint has integer coordinates. The logic based on the parity of x and y coordinates confirms that the argument holds true under all circumstances.
Examples & Analogies
Similar to how every party has at least one couple dancing together, our point selection ensures that some pairs will consistently align to create a midpoint that 'fits' perfectly on our coordinate grid.
--
Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Pigeonhole Principle:
States that if more points than categories, at least one category must contain multiple points.
- Midpoint Formula:
Method for calculating the average position of two points in a coordinate plane.
- Integer Coordinates:
Points defined by whole number values for x and y.
- Even/Odd Characteristic:
Classification of numbers that defines their divisibility by 2.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
If we take the points (1,2), (2,4), (3,1), (4,3), and (5,5), we can see that the midpoints for pairs like (1,2) and (3,1) both yield integer coordinates.
Consider points like (0,0), (2,2), (4,4), (1,1), and (3,3). The pairs (0,0) and (2,2) provide an integer midpoint of (1,1).
Memory aids
Imagine five friends trying to sit at four chairs at a party; someone must share a seat, just like our points with the same characteristic!
Flash Cards
Glossary
Distinct points
Points in a coordinate system that have different positions, thus carrying different coordinates.
Midpoint
The point that is equidistant from both endpoints in a line segment, calculated as the average of their coordinates.
Pigeonhole Principle
A principle stating that if more items are distributed than containers, at least one container must hold more than one item.
Integer coordinates
Coordinates in a plane that are whole numbers (positive or negative) without fractions or decimals.