Coplanar – Definition With Examples
10 minutes read
Created: December 23, 2023
Last updated: January 14, 2024
Welcome to another exciting post from Brighterly, your trusted partner in learning mathematics in a fun, engaging, and understandable way. Today’s topic is the concept of Coplanar, a fundamental aspect of geometry. This principle is essential to a wide range of studies, including engineering, computer graphics, and even architecture. Just as the world we live in is threedimensional, understanding the properties of points and lines in planes is crucial to our understanding of space.
Throughout this article, we’ll break down this seemingly complex concept into digestible parts. We’ll start with the definition of coplanar and what it means in the world of geometry. From there, we’ll explain the difference between collinear and coplanar, define coplanar and noncoplanar points, and explain coplanar and noncoplanar lines in geometry. Afterward, we’ll guide you through how to determine whether four given points or two given lines are coplanar.
What Does Coplanar Mean in Geometry?
When you hear the term coplanar in geometry, it refers to points or lines that lie on the same geometric plane. These planes are invisible, flat surfaces that extend indefinitely in every direction. Even though we can’t physically see these planes, they’re a crucial part of understanding spatial relationships in mathematics. This concept is one of the key pillars of geometry, making it possible to visualize and solve complex geometric problems.
What Is the Difference Between Collinear and Coplanar?
Now, let’s talk about collinearity and coplanarity. You might be wondering: are these just two fancy ways of saying the same thing? Not quite. Collinearity refers to the condition where three or more points lie on the same straight line. Coplanarity, on the other hand, allows for a broader range: points or lines can be in any arrangement as long as they’re on the same plane.
What Are Coplanar and Noncoplanar Points?
When discussing coplanarity, we often encounter the terms coplanar and noncoplanar points. As we learned earlier, coplanar points lie on the same plane. For example, the four corners of a square are coplanar because they rest on the same flat surface. Conversely, noncoplanar points don’t share a common plane. If you imagine a cube, the eight corners represent noncoplanar points because they occupy different planes.
What Are Coplanar and Noncoplanar Lines in Geometry?
Just like points, lines can be coplanar or noncoplanar too. Picture a notebook page filled with lines. All these lines exist on the same plane: the page surface. So, they are coplanar lines. Now, think about the lines that form the edges of a cube. They don’t all lie on the same plane, which makes them noncoplanar lines.
Coplanar Points Definition in Geometry
Coplanar points in geometry are defined as three or more points that lie on the same plane. Remember, a plane is an infinitely large, flat surface. Therefore, no matter how far apart they are, if points share a plane, they’re coplanar.
Non Coplanar Points Definition in Geometry
On the flip side, we have noncoplanar points. These are points that don’t share a common plane. Imagine a pyramid. Each corner represents a point, and since these corners don’t all lie on the same plane, they are noncoplanar points.
How to Determine Whether Given 4 Points are Coplanar?
It might sound complex, but determining if four points are coplanar isn’t as tricky as it seems. You just need the coordinates of the points. We can determine coplanarity by checking the volume of the parallelepiped formed by the vectors of these points. If the volume equals zero, the points are coplanar.
Coplanar and Non Coplanar Lines
Again, we distinguish between coplanar and noncoplanar lines based on whether they lie on the same plane. The lines forming a square, for example, are coplanar. The edges of a cube, however, are noncoplanar lines because they exist on different planes.
How to Determine Whether Given 2 Lines are Coplanar?
To check if two lines are coplanar, you need their vector representations. If the determinant of their direction ratios is zero, the lines are coplanar. It might sound complex, but this concept becomes much easier with practice!
Condition For Coplanarity of Lines in Vector Form
The condition for coplanarity of lines in vector form is that the determinant of their direction ratios is zero. This is a key concept in vector algebra and helps solve many geometric problems.
Condition For Coplanarity of Lines in Cartesian Form
In Cartesian form, the condition for coplanarity is similar. If the determinant formed by the direction ratios of the lines equals zero, the lines are coplanar. This forms a basis for understanding many advanced geometry problems.
Important Notes on Coplanar
While we’ve covered a lot, here are some important notes on coplanar. Remember, any two points are always coplanar since they can exist on an infinite number of planes. Also, any three noncollinear points determine a unique plane, making them coplanar.
Solved Examples on Coplanar
Understanding the concept of coplanar is easier with practical examples. Let’s look at some solved examples on coplanar to solidify your understanding.

Example 1: We have points A(1,2,3), B(4,5,6), C(7,8,9), and D(2,3,1). Are these points coplanar?
Solution: To find out if the points are coplanar, we calculate the volume of the parallelepiped formed by these points. We use the formula:
V = [AB, AC, AD] = [(41)i + (52)j + (63)k] . [(71)i + (82)j + (93)k] X [(21)i + (32)j + (13)k]
Simplifying this, we find that V = 0. Therefore, points A, B, C, and D are coplanar.

Example 2: Given two lines represented by vectors: Line 1: r = (2i + 3j + 4k) + λ(2i + 3j + 4k) Line 2: r = (3i + 4j + 5k) + μ(2i + 3j + 4k)
Are these lines coplanar?
Solution: To check for coplanarity, we calculate the determinant of the direction ratios.
D =  2 3 4   3 4 5   2 3 4 
D = 0
Hence, the two lines are coplanar.
Practice Problems on Coplanar
Now that we’ve seen some examples, let’s try some practice problems on coplanar. These exercises will help reinforce your understanding of the concept.
 Problem 1: Determine whether the points E(3,4,5), F(6,7,8), G(9,10,11), and H(4,5,2) are coplanar. Use the same method as in Example 1.
 Problem 2: Are the lines represented by the vectors coplanar? Line 1: r = (5i + 6j + 7k) + λ(2i + 3j + 4k) Line 2: r = (6i + 7j + 8k) + μ(3i + 4j + 5k)
Conclusion
In this comprehensive guide from Brighterly, we have delved into the concept of coplanar, dissecting its definition, importance, and relevance in geometry, while also clarifying its difference from collinearity. We also provided some solved examples to better illustrate the concept and gave you some practice problems to test your understanding.
At Brighterly, we are committed to making learning mathematics as accessible, enjoyable, and interactive as possible. We hope this guide has helped you grasp the concept of coplanar and how it works in geometry. As with any other mathematical concept, the key to understanding coplanar is constant practice.
Remember, mathematics is not about memorizing formulas and methods; it’s about understanding concepts and their applications. So, keep practicing, stay curious, and continue exploring the fascinating world of geometry with Brighterly!
Frequently Asked Questions on Coplanar
What are coplanar points?
Coplanar points refer to points that exist on the same geometric plane. In simpler terms, if you could connect the points with a flat sheet, without any point sticking out, they are considered coplanar.
How can I tell if points are coplanar?
To determine if points are coplanar, you can calculate the volume of the parallelepiped (a 3D shape) formed by vectors originating from these points. If the volume equals zero, then the points are coplanar.
Are any two lines always coplanar?
Not always. Two lines are coplanar if they exist on the same plane. This can occur if the lines are parallel, intersecting, or if they are the same line. However, if the lines are skew lines (nonparallel lines that don’t intersect), they are not coplanar.
What’s the difference between collinear and coplanar?
Collinear refers to points that lie on the same straight line, while coplanar refers to points that lie on the same geometric plane. Therefore, all collinear points are coplanar, but not all coplanar points are collinear.
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