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Learn Geometry with GeoGebra

GeoGebra is a free dynamic mathematics software and it is especially created for teaching and learning. It is an open-source software available for multiple platforms (iOS, Android, Windows, Mac OS, and Linux) and it has been translated into many languages. It has an easy-to-use interface and yet many powerful features.

GeoGebra is both an online web application and an online downloadable application.

Drawing lines and segments

You only need to use the appropriate tools in the Toolbar to create geometric constructions on the Graphics View with your mouse. For example, select Line, click twice on the Graphics View and you will get two points (A, B) and a line.

If you want to draw segments, select the Segment option on the Line menu. Click again twice on the Graphics View and you will get a segment.

Drawing lines and segments with Geogebra

Drawing lines and segments with Geogebra

It is also possible to get the same results from the input field in the bottom panel. If you want to introduce points, just type: A= (1, 6). B = (3, 2). Drawing lines is very simple, too. Let’s draw the line that passes through two points. Let’s find its slope = (ya-yb)/(xa-xb) = (6-2)/(1-3)=-2. Now we can write its equation: y-ya = slope*(x-xa), (y-6)=-2*(x-1), (y-6)=-2x+2, y=-2x+2+6, so you need to type, y=-2x+8.

Draw a line (A, B) and a point (C). Let’s construct _the parallel line to our initial line which passes through C. Select the Parallel Line option on the Perpendicular Line menu. Click on C, and then on our previous line (A, B).

Drawing lines and segments with Geogebra

Drawing lines and segments with Geogebra

Let’s construct the perpendicular line to our initial line which passes through C. Just select Perpendicular Line, click on C, and then on our previous line (A, B). The algebra view on the left panel allows you to see and edit all created objects. For instance, point B and its two coordinates.

Next, we will measure the length of a segment. Draw two points (A, B) and a segment (AB). Select Angle, Distance or Length, and click on the segment. You can also type commands in the Input field and get the same results. For example, d = Distance [A, B] calculates the length of the segment AB; PerpendicularLine[C, a] creates a line through the point C perpendicular to the given line (a); and Line[C, a] creates a line through the point C parallel to the given line (a).

Let’s calculate the centre of our segment (AB). Select the Midpoint or Center option on the Point menu, and click on the segment. Alternatively, you can introduce the command Midpoint [A, B].

Drawing angles, circles, tangents, and bisectors.

Let’s draw an angle and its bisector.

Let’s draw a circle.

Select Circle with Center Through Point in the Toolbox and then, click on one point in the centre panel (centre, A) and then another (one point in the circle, B). As always, you can construct circles by typing myCircle = Circle [A, B] in the input field.

Given a circle and a point C, we will draw the two tangent lines to our circle which pass through C. Select Perpendicular Line, Tangents, and click on point C and the circle or just type, Tangent [myCircle, C].

Tangent lines to our circle

Tangent lines to our circle

Let’s construct the midpoint of a segment

Midpoint of a segment

Midpoint of a segment

Observe that this line passes through our Midpoint C, and by doing so, it confirms that C is the midpoint of our original segment AB.

Calculate the area of a circle

user@pc:~$python
Python 3.8.6 (default, May 27 2021, 13:28:02) 
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information
import math
math.pi * pow(2.96, 2)
27.52537819369233
Area of a circle

Area of a circle

Working with Triangles

Working with triangles

Working with triangles

The Circumscribed circle

The circumscribed circle of a polygon is a circle that passes through all the vertices of the polygon. Its center is called the circumcenter. Not every polygon has a circumscribed circle. A polygon that does have one is called a cyclic polygon. All triangles are cyclic.

The Circumscribed circle

The Circumscribed circle

The incircle or inscribed circle

The incircle or inscribed circle of a triangle is the largest circle contained in the triangle. It touches or is tangent to the three sides. The center of the incircle is called the triangle’s incenter.

The incircle

The incircle

Translations

There are three basic transformations: translations, reflections, and rotations. These transformations move the figure or object without making any changes to its shape and size. A reflection flips an object over to create a mirror image, a rotation turns an object, and a translation moves or slides an object to a different location without rotating or resizing it. I want to insist again that after any of these transformations (flip, turn or slide) the figure or object still has the same size, area, angles, and line lengths.

Calculate the area (Angle, Area) and the triangle’s interior angles. When you translate something, you’re simply moving it around. You don’t distort it in any way. If you translate a segment, its length doesn’t change. Similarly, if you translate an angle or a triangle, the measure of the angle or the triangle’s area do not change.

translation

translation

Rotations in Geogebra

Using LaTex with Overleaf

Using LaTex with Overleaf

Rotations in Geogebra

Rotations in Geogebra

Calculate the area (Angle, Area) and the square’s interior angles. When you rotate something, you’re simply whirling it around. You don’t distort it in any way and its area does not change.

Reflection

A reflection can be thought of as flipping an object. An object and its reflection do have the same shape and size, but they appear as if they are mirror reflections. Every point is the same distance from the central or the mirror line.

Reflections with Geogebra

Reflections with Geogebra

Next, we will reflect the triangle across the y-axis, so the line d is x = 0. Observe that the y-coordinate remains the same, unaltered but the x-coordinate is negated (transformed into its opposite) A’ = (-x, y) = (-2.58, 4.18).

Reflections with Geogebra

Reflections with Geogebra

Let’s reflect the triangle across the line y = x. When you do so, the x-coordinate and y-coordinate swap or change places A’ = (y, x) = (4.18, 2.58).

Reflections with Geogebra

Reflections with Geogebra

Finally, draw a pentagon (Polygon, Regular Polygon, poly1), a point (New Point, F(0,0)), and reflect the pentagon over the origin by selecting from the menu Reflect Object in Line, the option Reflect Object in Point, and clicking on the pentagon and the point F. Observe that both the x-coordinate and the y-coordinate are negated (transformed into their opposite), so A’ = (-x, -y) = (-2.73, -3.28)

Reflections with Geogebra

Reflections with Geogebra

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