GeoGebra Tutorial 1 – Quadrilaterals & Midpoints

If this is your first time to use GeoGebra, I suggest that you read Introduction to GeoGebra which contains the discussion about the basics of GeoGebra and parts of the GeoGebra window. The assumption in this tutorial is that you are already familiar with the parts of the GeoGebra window.

In the tutorial below, menu commands, located in the menu bar, are in brown bold text, and submenus are denoted by the > symbol. For example, Options>Labeling> New Points Only means, click the Options menu, choose Labeling from the list, then select New Points Only. The GeoGebra tools are denoted by orange texts. For example, New Point means the new point tool.

Problem: Investigate what happens if you connect the the consecutive midpoints of a quadrilateral.

In this tutorial, we use GeoGebra to explore the properties of the midpoints of a quadrilateral. In doing this tutorial, you will learn how to use the following tools:  Move, Midpoint or Center, Segment between Two Points and Polygon.  If you want to view the output of this tutorial, click here.

1.) Open GeoGebra and select Algebra from the Perspective menu .
newpointpng 2.) To construct the vertices of the quadrilateral, click the New Point tool, and then click four distinct places on the Graphics view.
movepng 3.) If the labels of the points are not displayed, click the Move tool, right  click each point and click Show label from the context menu.
segmentpng 4.) Select the Segment tool and click point A and click point B two distinct points to construct segment AB. To construct BC, with the Segment tool still active, click point B and another location to create segment BC. Now, construct CD and AD to complete the quadrilateral. Your drawing should look like Figure 1.


Figure 1 – Quadrilateral ABCD


5.) Click the Move tool and move the vertices of the quadrilateral. What do you observe?
midpointpng 6.) To determine the midpoint of each side of the quadrilateral, choose the Midpoint or Center and then click the segments (not the points) in the following order: AB, BC, CD, and AD


7.) Select the Move tool, right click the midpoints and click Show label from the context menu. After that step, your drawing should look like Figure 2.


Figure 2 – Quadrilateral ABCD with Midpoints E, F, G and H.


8.) Move the vertices of the quadrilateral. What do you observe?
polygonpng 9.) To have a better view, connect the midpoints of the quadrilateral using the Polygon tool. To do this, click the Polygon tool and click the points in the following order: Point E, point F, point G, point H and then point E again to close the polygon.


10.  Move the vertices of the quadrilateral. What do you observe about the figure?
11.  What conjecture can you make based on your observation?

Updated: 11 January 2016 (GeoGebra 5.0)

Introduction to GeoGebra

GeoGebra is a dynamic mathematics software that integrates algebra, geometry, calculus, and statistics. It is capable of displaying data simultaneously: numerically, graphically, and algebraically. GeoGebra was created by Markus Hohenwarter in 2001. It has become an open-source software and is continually improved by programmers, mathematicians, and mathematics teachers around the world.

The  GeoGebra Window

The GeoGebra 5.0 window is shown in the first figure.  The leftmost pane is the Algebra view, where algebraic descriptions of objects on the Graphics view are displayed.  The Graphics view is where constructions, drawings, and graphs are displayed. By default, the Coordinate axes are displayed when you open GeoGebra.

geogebra 5.0

The uppermost part of the window is the Menu Bar. The Menu Bar is used for managing files, editing files, and modifying settings.  Below the menu bar is the Toolbar where tools for drawing, constructing, measuring, and manipulating objects are shown. The Input Bar is used for typing equations, algebraic commands, and computations.

The coordinate axes, the algebra window and the input bar can be displayed or hidden using the View menu.

To know more about GeoGebra, you can browse the Help menu after finishing this tutorial.

The Toolbar and the Tools

The Toolbar contains the tools that are used to construct objects such as points, lines and other figures. Shown below are the categories of tools and the default tools displayed.

In the diagram above, the Move tool is highlighted by a blue border which means that it is the active tool.  As long as a certain tool is active, it will construct the same drawing or perform the same task. You do not need to click it every time you construct the same object.

The icon of each tool has a triangular arrow  located at the bottom-right of the tool that you can click if you want to display other tools.  The third figure displays the tools if you click the Line through two points button.


If you want to draw a Ray through two points, you just have to click the arrow at the Line through two points tool then click it as shown above. The line icon will be replaced by the ray icon, which means that the latter is now active.

Now, we learn the basic of GeoGebra in the first tutorial, Locating the Triangle’s Centroid.  You can view the rest of tutorials in the GeoGebra Essentials page and the GeoGebra main page.

Last Update: 16 October 2015, GeoGebra 5.0

Sum of the interior angles of a polygon

We were taught that if we let m be the angle sum (the total measure of the interior angles) and n be the number of vertices (corners)  of a polygon, then m = 180(n-2).  For example, a quadrilateral has 4 vertices, so its angle sum is 180(4-2) = 360 degrees.  Similarly, the angle sum of a hexagon (a polygon with 6 sides) is 180(6-2) = 720 degrees.

But where did this formula come from?  Does this formula work for all polygons?  (Note that in this discussion, when we say polygon, we only refer to convex polygons).

Before we answer these questions, let us first have a brief review of some elementary concepts.

Polygons and Interior Angles

A polygon is a closed figure with finite number of sides. In the figures below, ABCDE is a polygon with 5 sides and (5 vertices).  It is clear that the number of sides of a polygon is always equal to the number of its vertices.

A polygon has interior angles.  In the first figure below, angle B measuring 91 degrees is an interior angle of polygon ABCDE. The angle sum m of ABCDE (not drawn to scale) is given by the equation

m = 126 + 91 + 113 + 102 + 108 = 540 degrees.


In the second figure, if we let a_1, a_2 and a_3 be the measure of the interior angles of triangle ABE, then the angle sum m of triangle ABE is given by the equation m = a_1 + a_2 + a_3.

Angle Sum

To generalize our calculation of  angle sum, we use the fact that the angle sum of a triangle is 180 degrees. Notice that any polygon maybe divided into triangles by drawing diagonals from one vertex to all of the non-adjacent vertices.  In the second figure above, the pentagon was divided into three triangles by drawing diagonals from vertex E to the non-adjacent vertices B and C forming BE and CE.  Now let a_k, b_k and c_k, where k = 1, 2, 3 be measures of the interior angles of the three triangles as shown on the second figure.


Calculating the angle sum of pentagon ABCDE we have


Notice that the angle measures in the first line of our equation is just a rearrangement of the measures of the interior angles of the three triangles. Hence, the angle sum of the pentagon is equal to the angle sum of the three triangles. Therefore, we can conclude that the sum of the interior angles of a polygon is equal to the angle sum of the number of triangles that can be formed by dividing it using the method described above. Using this conclusion, we will now relate the number of sides of a polygon, the number of triangles that can be formed by drawing diagonals and the polygon’s angle sum.


From the table above, we observe that the number of triangles formed is 2 less than the number of sides of the polygon.  This is true, because n - 2 triangles can be formed by drawing diagonals from one of the vertices to n - 3 non-adjacent vertices. Therefore, there the angle sum m of a polygon with n  sides is given by the formula

m = 180(n - 2)

A More Formal Proof

Theorem: The sum of the interior angles of a polygon with n sides is 180(n-2) degrees.


Assume a polygon has n sides. Choose an arbitrary vertex, say vertex V.  Then there are n - 3 non-adjacent vertices to vertex V.  If diagonals are drawn from vertex V to all non-adjacent vertices, then n - 2 triangles will be formed.  The sum the interior angles of n -2 triangles is 180(n - 2). Since the angle sum of the polygon with n sides is equal to the sum the interior angles of n - 2 triangles, the angle sum of a polygon with n sides is 180(n-2). \blacksquare


1.)    Find the number the angle sum of a dodecagon (12-sided polygon).

2.)    The angle sum of a polygon is 3240 degrees. What is the number of its sides?

3.)    The measure of one of the angles of a regular polygon is 150. Find its number of sides.

4.)    From this, prove that the sum of the interior angles of a polygon is 360 degrees.


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