Namaste, future engineers! Welcome to the exciting world of Three Dimensional Geometry. Today, we're going to start with a very fundamental concept that you've likely encountered before, but we'll elevate it to a whole new dimension (pun intended!). We're talking about the
Distance between two points.
Think about it, whether you're planning a trip, playing a video game, or even just walking from your bed to your study table, you're constantly dealing with distances. In mathematics, we just formalize this concept.
### 1. The Basics: Distance on a Number Line (1D)
Let's start super simple, shall we? Imagine a straight road, and on this road, there are houses numbered according to their distance from a starting point. This is like a number line!
Suppose you have two houses:
* House A is at position $x_1 = 2$.
* House B is at position $x_2 = 7$.
How far apart are they? You'd simply subtract their positions, right? $7 - 2 = 5$ units.
What if House A was at $x_1 = -3$ and House B was at $x_2 = 4$? The distance would be $4 - (-3) = 4 + 3 = 7$ units.
What if you subtracted in the other order? $-3 - 4 = -7$. But distance can't be negative! That's why we always take the
absolute value.
So, the distance between two points $P(x_1)$ and $Q(x_2)$ on a number line is given by:
Distance $PQ = |x_2 - x_1|$ or $|x_1 - x_2|$
It's straightforward, isn't it? Just the difference between their coordinates.
### 2. Stepping Up: Distance in a Plane (2D)
Now, let's add a bit more complexity. Imagine you're looking at a map. You don't just move along a straight line; you can move left, right, up, and down! This is our familiar 2D coordinate plane, defined by an X-axis and a Y-axis.
Suppose you want to find the distance between two points, say:
* Point $A(x_1, y_1)$
* Point $B(x_2, y_2)$
How do we do this? We can't just subtract X's and Y's separately. The key here is our old friend, the
Pythagoras Theorem!
Let's visualize this:
1. Plot points $A(x_1, y_1)$ and $B(x_2, y_2)$ on a graph.
2. Draw a horizontal line from $A$ and a vertical line from $B$ such that they intersect at a new point, say $C$.
3. What are the coordinates of $C$? It will have the x-coordinate of $B$ and the y-coordinate of $A$, so $C(x_2, y_1)$.
4. Now, look at the triangle $ riangle ABC$. It's a
right-angled triangle with the right angle at $C$.
* The length of side $AC$ (horizontal distance) is the distance between $(x_1, y_1)$ and $(x_2, y_1)$, which is $|x_2 - x_1|$.
* The length of side $BC$ (vertical distance) is the distance between $(x_2, y_1)$ and $(x_2, y_2)$, which is $|y_2 - y_1|$.
* The distance $AB$ is the hypotenuse!
According to Pythagoras Theorem ($hypotenuse^2 = base^2 + height^2$):
$AB^2 = AC^2 + BC^2$
$AB^2 = (|x_2 - x_1|)^2 + (|y_2 - y_1|)^2$
Since squaring a number makes it positive, we can drop the absolute value signs:
$AB^2 = (x_2 - x_1)^2 + (y_2 - y_1)^2$
Therefore, the distance between two points $P(x_1, y_1)$ and $Q(x_2, y_2)$ in a 2D plane is:
Distance $PQ = sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}$
This is the famous
2D Distance Formula. It's super important!
Example 1:
Find the distance between points $A(1, 2)$ and $B(5, 5)$.
Solution:
Here, $x_1 = 1, y_1 = 2$ and $x_2 = 5, y_2 = 5$.
$Distance = sqrt{(5 - 1)^2 + (5 - 2)^2}$
$Distance = sqrt{(4)^2 + (3)^2}$
$Distance = sqrt{16 + 9}$
$Distance = sqrt{25}$
$Distance = 5$ units.
### 3. Into the Third Dimension: Distance in Space (3D)
Alright, now for the main event! What happens when we add one more dimension? Instead of a map, imagine you're a bird flying in the sky, or looking at stars in the universe. You don't just have X and Y coordinates; you also have a Z-coordinate, representing depth or height!
So, in 3D space, a point is defined by three coordinates: $(x, y, z)$.
Let's say we have two points:
* Point $P(x_1, y_1, z_1)$
* Point $Q(x_2, y_2, z_2)$
How do we find the distance between them? Good news! The logic is exactly the same, an extension of the Pythagoras Theorem.
Imagine constructing a rectangular box (a cuboid) with $P$ and $Q$ as diagonally opposite vertices.
1. First, project these points onto the XY-plane. Let $P'(x_1, y_1, 0)$ and $Q'(x_2, y_2, 0)$ be these projections. The distance $P'Q'$ is the 2D distance we just learned: $sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}$.
2. Now, imagine a point $R$ with coordinates $(x_2, y_2, z_1)$. This point $R$ lies in the same plane as $P$ (the plane $z=z_1$) but directly "below" or "above" $Q$ in the $xy$-plane.
3. Consider the triangle formed by $P$, $R$, and $Q$.
* The distance $PR$ is the 2D distance between $P(x_1, y_1, z_1)$ and $R(x_2, y_2, z_1)$. Since their z-coordinates are the same, this is effectively a 2D distance in the plane $z=z_1$. So, $PR = sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}$.
* The distance $RQ$ is the vertical distance between $R(x_2, y_2, z_1)$ and $Q(x_2, y_2, z_2)$. This is simply $|z_2 - z_1|$.
* The triangle $ riangle PRQ$ is a
right-angled triangle at $R$. (Think of $PR$ as lying flat on a table, and $RQ$ as a pole sticking straight up from the table).
Applying Pythagoras Theorem to $ riangle PRQ$:
$PQ^2 = PR^2 + RQ^2$
Substitute the values:
$PQ^2 = left( sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}
ight)^2 + (|z_2 - z_1|)^2$
$PQ^2 = (x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2$
Finally, taking the square root, we get the
3D Distance Formula:
Distance $PQ = sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2}$
See how beautifully it extends? Each coordinate difference is squared, added together, and then the square root is taken. It's a direct generalization of the 2D formula!
### Important Points & Tips:
* The order of subtraction for $x, y,$ or $z$ coordinates doesn't matter because we square the difference. $(x_2 - x_1)^2$ is the same as $(x_1 - x_2)^2$.
* The distance is always a non-negative value.
* This formula is fundamental to 3D geometry. You'll use it to find lengths of line segments, check if points are collinear, identify types of triangles, and much more!
*
JEE Focus: While the formula itself is basic, JEE problems often involve using this formula in conjunction with other concepts like properties of triangles, collinearity, or even to find the locus of a point. So, ensure your calculations are quick and accurate.
Example 2:
Find the distance between the points $A(1, -3, 4)$ and $B(-4, 1, 2)$.
Solution:
Here, $x_1 = 1, y_1 = -3, z_1 = 4$ and $x_2 = -4, y_2 = 1, z_2 = 2$.
Using the 3D distance formula:
$AB = sqrt{((-4) - 1)^2 + (1 - (-3))^2 + (2 - 4)^2}$
$AB = sqrt{(-5)^2 + (1 + 3)^2 + (-2)^2}$
$AB = sqrt{(-5)^2 + (4)^2 + (-2)^2}$
$AB = sqrt{25 + 16 + 4}$
$AB = sqrt{45}$
To simplify the square root, we look for perfect square factors: $45 = 9 imes 5$.
$AB = sqrt{9 imes 5} = 3sqrt{5}$ units.
Example 3:
Calculate the distance of the point $P(3, -2, sqrt{3})$ from the origin $O(0, 0, 0)$.
Solution:
Here, $x_1 = 3, y_1 = -2, z_1 = sqrt{3}$ and $x_2 = 0, y_2 = 0, z_2 = 0$.
$OP = sqrt{(0 - 3)^2 + (0 - (-2))^2 + (0 - sqrt{3})^2}$
$OP = sqrt{(-3)^2 + (2)^2 + (-sqrt{3})^2}$
$OP = sqrt{9 + 4 + 3}$
$OP = sqrt{16}$
$OP = 4$ units.
Quick Tip: The distance of a point $(x, y, z)$ from the origin $(0,0,0)$ is simply $sqrt{x^2 + y^2 + z^2}$. This is a common shortcut!
Dimension |
Number of Coordinates |
Distance Formula (Points $P(X_1, ...)$ and $Q(X_2, ...)$) |
|---|
1D (Number Line) |
1 (x) |
$|x_2 - x_1|$ |
2D (Plane) |
2 (x, y) |
$sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}$ |
3D (Space) |
3 (x, y, z) |
$sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2}$ |
This progression from 1D to 3D beautifully illustrates how mathematical concepts are often built upon simpler ones, extending their logic to higher dimensions. Master this 3D distance formula, and you've unlocked a powerful tool for exploring the geometry of our world! Keep practicing, and you'll find it second nature.