Centroid: Definition, Theorem & Formula

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  • 0:00 Definition Of The Centroid
  • 1:14 Properties Of The Centroid
  • 3:09 Problems Using The Centroid
  • 5:40 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth Often

Elizabeth has taught high school math for over 10 years, and has a master's in secondary math education.

Have you ever wondered how to find the balancing point of a triangle? The answer can be found in this lesson on the centroid of a triangle! We'll learn how medians are used to locate the centroid and then discuss its various properties.

Definition of the Centroid

The centroid of a triangle is the point where the three medians of the triangle intersect. The medians are the segments that connect a vertex to the midpoint of the opposite side. In this image, point G is the centroid of triangle ABC.

To locate the centroid of a triangle, it's easiest to draw all three medians and look for their point of intersection. To draw the median of a triangle, first locate the midpoint of one side of the triangle. Draw a line segment that connects this point to the opposite vertex. A triangle with one median drawn is shown here:

Repeat this process for the other two sides of the triangle. Once you have drawn all three medians, locate the point where all three intersect. This point is the centroid.

For a triangle made of a uniform material, the centroid is the center of gravity. Imagine that an artist designed a sculpture in which multiple metal triangles were balanced at the ends of pieces of wire. In order for the triangles to be perfectly balanced, the wires would have to be attached at the center of gravity, or centroid, of each triangle, like in the mobile shown below.

Properties of the Centroid

So what are some properties of a centroid?

The centroid is always located inside the triangle. This is true whether the triangle is acute, right, or obtuse. In this image, you can see that the centroid is inside of each the triangles, even though they all have different angle measures.

Another important property of the centroid is that it is located 2/3 of the distance from the vertex to the midpoint of the opposite side. Another way of saying this is that the centroid divides the median in a 2:1 ratio. The distance from the centroid to the vertex is twice as long as the distance from the centroid to the midpoint of the side opposite the vertex.

You can use the distance from the centroid to the vertex of a triangle to find the length of the entire median segment. You can also use this property to locate the centroid in a triangle when you can only draw one median.

So in this image, if you're told that the length of segment CF is 12 and that CF is a median of the triangle, where would the centroid, G, be located? We can use the 2/3 rule to determine how far from vertex C the centroid will be. We can use a simple equation to find the answer:

The centroid will be 8 units from vertex C, along the segment CF.

A landscape designer could use this same property if she creates a courtyard on a triangular shaped lot. She wishes to put a sculpture in the middle of the courtyard. As can be seen in this image, the three paths through the courtyard each go from a vertex to the midpoint of the opposite side. The statue will be placed at the centroid of the triangle. By using the properties of the centroid, the landscape designer can determine the distance from each vertex of the triangle to the statue.

Problems Using the Centroid

In the math classroom, nearly all problems using the centroid will also involve one or more medians in the triangle. We'll do some examples using this image as a reference:

Question: If segment CG has a length of 6 units and G is the centroid of the triangle, how long is segment CF?

Solution: Since CG is the segment joining the vertex to centroid, it is 2/3 of the total length of CF. Dividing by 2/3 will give us the length of the full segment. We can write an algebra equation and solve.

CF is 9 units long.

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