1. Chapter 10
Constructions

2. 10.1 What Construction Means

3. What is a construction ?
In geometry there is a difference between drawing a geometric picture and doing a geometric construction.
The main difference is in the tools used for each method.

4. What is a construction ? Constructions can only use these three
Drawings allow any tools to be used:
Pens
Pencils
Rulers
Protractors
Compasses
Etc…
Constructions can only use these three
Pencil
Straightedge
compass

5. What can be constructed ?
Line Segments
Arcs
Circles
Triangles
Polygons
Parallel Lines
Perpendicular Lines
Any Geometric Shape….

6. Using a compass
A compass has two arms which are jointed at the top.

7. Using a compass
One arm ends in a point and the other arm holds a pencil

8. Using a compass
The arms can be adjusted so that the distance between the point and the pencil is bigger or smaller.

9. When drawing with a compass...
It is important that you hold the compass in one of these ways:
Place the point of the compass on the paper. Then hold only the small part above where the arms meet to spin the compass and to draw an arc or circle. OR

10. When drawing with a compass...
2) Place the point of the compass on the paper. Then turn the compass by holding the arm that has the point of the compass and let the pencil move over the paper to draw the arc or circle. OR

11. When drawing with a compass...
3) Place the point of the compass on the paper. Then hold only the small part above where the arms meet so that both the point and the pencil softly touch the paper. Then use your other hand to slowly turn the paper underneath the compass so that the pencil can trace out the arc, or circle, on the moving paper.

12. What do line segments, arcs and angles look like?
A line segment had two endpoints that are connected by a straight line.

13. What do line segments, arcs and angles look like?
An arc is a part of a circle.

14. What do line segments, arcs and angles look like?
An angle is two rays that are joined together at their endpoints.

15. What does it mean to “measure” when doing a construction?
In constructions, measure never means to use a ruler to measure length, nor does it mean to use a protractor to measure an angle.

16. What does it mean to “measure” when doing a construction?
When the instructions for a construction include the word “measure”, this requires the use of a compass.
Compass width is how far apart the pencil point and the point on the other arm of the compass are.

17. Length of a segment
To measure the length of a segment, put the point of the compass on one end of the segment and stretch the compass until the pencil point meets the other end of the segment.

18. Arc
To measure an arc, put the point of the compass at one intersection point on the arc and stretch the compass until the pencil point meets the other intersection point on the arc.

19. Following Directions Matters !
Once you learn the basics of doing construction, you will see similarities in what you are asked to do in each set of instructions.

20. Here are some common directions and what they should look like
Draw a line with the straightedge

21. 2) Put the compass on the endpoint and make a small arc that intersects the segment.

22. 3) Draw an arc that intersects both sides of the angle.

23. Should you erase all those marks?
A completed constructions has dark pencil marks and light compass marks.

24. Should you erase all those marks?
Sometimes there can be a lot of overlapping arcs and lines
You might have wanted to draw a square and ended up with some of the sides extending past the edge of the square.
That’s OK

25. Should you erase all those marks?
DO NOT erase the extra marks that are part of doing the construction.
If you erase, it looks like a drawing, not a construction.

26. Helpful Hints
Once you have measured a distance with a compass, make sure that you hold the compass carefully so that you do not pull the arms farther apart or push them closer together before you do the next step.

27. Helpful Hints Do not push too hard on the paper.
If you find that your pencil is making a rough dark line
you are pushing the compass down too hard.
If you see a big hole in your paper from the compass point
you are pushing too hard on the compass, or you are pulling on the paper.
BE GENTLE

28. Helpful Hints If your compass seems to fall closed or open easily
Tighten the screw where the arms are joined if your compass has one.
If you can’t tighten the screw you might need a new compass.
If you are having trouble doing part of a construction, try turning the paper so that it faces a different direction.

29. Helpful Hints
Use a separate pencil to draw lines, instead of trying to use the pencil in your compass.
You might accidentally change the width
Use colored pencils

30. Construction #1: Copy a Segment
Given a segment, construct a segment congruent to the given segment.
Step 1: Use the straightedge to draw a line and label it l A B
Put compass point here
Step 2: Choose any point on l and label it C.
Step 3: Set the compass at A and make an arc at B.
Step 4: Using the same opening, set your compass at C and make an identical arc so that it intersects line l. Label this point D. l C D
Put compass point here

31. Construction #2: Copy an Angle
Given a angle, construct an angle congruent to the given angle.

32. Construction #2: Copy an Angle
Step 1: You need to start with a line l and a point F which corresponds to C. C E
Step 2: Opening your compass to any comfortable distance, set the compass at C and make an arc that intersects both sides of the angle. Label these points E and D.
Put compass point here D
Put compass point here
Step 3: Using the same opening, set the compass at F and make an identical arc which intersects the line l. Label this point G.
Put compass point here
Step 4: On the original angle, set the compass at D and make an arc that intersects the other side of the angle at E. F l G

33. Construction #2: Copy an AngleD E
Step 5: Using the same opening, set your compass at G and make an identical arc so that it intersects the arc from step 3. Label this point H.
Step 6: Draw H F G
Put compass point here

34. Given an angle, construct the bisector of the angle?
Construction #3
Given an angle, construct the bisector of the angle? C A B Z
Given: X Y
Procedure:
Using B as center and any radius, draw
and arc that intersects BA at X and BC at point Y.
2. Using X as center and a suitable radius, draw and arc.
Using Y as center and the same radius, draw an arc
that intersects the arc with center X at point Z.
3. Draw BZ.