# Parallelism

## Parallel Lines - Definition

Two lines are parallel if, and only if, they coincide (that is, equal) or are coplanar and have no common point.

## Question 1

Which pairs of lines are parallel? (use the "Show / Hide angle marks" box to help you)

Check all that apply

## Question 2

In the previous structure, which pairs of angles are congruent?

Check all that apply

## Question 3

In the previous structure, which of the following pairs of angles are supplementary?

Check all that apply

## Constructing a parallel line

In the following GeoGebra applet, follow the steps below: - Select the POINT (Window 2) and draw a point B on line r.      - Select the COMPASS tool (Window 6). Then click on point A and point B (it will open the compass) and again on point A (it will close the compass and form a circle). After that click on point B and point A (it will open the compass) and again on B (it will close the compass and form a second circle). - Select the INTERSECT  (Window 3) and mark the intersection C of the last circle with the line r.   - Select the COMPASS (Window 6). Then click on point C and point A (it will open the compass) and again on point B (it will close the compass and form a circle). - Select the option INTERSECT  (Window 3) and mark point D, which is the upper intersection of the first circunference with the third circunference.   -Select the option LINE (Window 3) and click on point A and point D. Label this line s. - Select the option SHOW / HIDE OBJECT (Window 7) and hide the circles, points B, C and D, leaving only the lines and point A. -Select the option RELATION (Window 8) and click on the two lines. What happens? - Select the option MOVE (Window 1) move point A or line r. What can you see?

## Analysis

Write an argument to justify the construction.

## Question 4

Move the selector "t". Also move the vertices of the triangle. What can you see?

## Question 5

Explain the previous property.