## Tuesday, 27 June 2017

### Geometrical Construction: Spot the Error (2)

As a group, watch the clip and try to spot as many errors as possible.

1. The compass should not be shortened

1. To describe clearly, should say the length of the compass (pointed end to the pencil) should not change when drawing the 2nd pair of arcs.

2. His second time drawing the arcs on both sides was not the same as the distance he used to extend his arms earlier. He also did not extend the arms all the way to further than the midpoint in the second time drawing.

3. He changed the distance between the arms of the compass, causing the distance A and the midpoint and B and the midpoint to be different

1. The first part of the sentence is correct.
The 2nd part was not clear. Think of a way to refine it.

4. The compass did not reach Point B.

1. Need to be clearer. What is the 'thing' that did not reach Point B?
It is necessary?
Watch the clip and examine how to place the compass to the construct the perpendicular bisector:

5. After drawing the first arc, he changed the length in between the arms to draw the second arc. He should not change the length in between the arms.

6. He should have extended the arms to the length of AB

1. What is one important thing to take note of when handling the compass (to create a bisector)?
Watch and examine how to handle the compass when drawing the perpendicular bisector:

7. The arms of the compass should be the same length as the line and not shorter.

8. The distance of the arc from both points are not the same.

9. the did not put the arms of the compass to the length of the smaller diagonal when he drew the second pair of arc.

1. The sentence (description) could be put across in a more succinct way. There is no diagonal involved here. You can say, the distance between the pointed end and the pencil of the compass has changed when he drew the 2nd pair of arcs.

10. The length between the arms on the first drawing is not the same as the length between the arms on the second drawing as he changed it. Thus the midpoint is not accurate.

11. He shortened the length of the arms when he was drawing the arc from point A.

1. Correct. The length should remain the same.

12. The compass should be stretched out at an equal distance when drawing the arc from point A and B. But he changed the length when drawing the arc from point B.

1. Correct.
In fact, you could say, the distance between the pointed end and the pencil should remain the same when drawing both pairs of arcs.

13. He should not have changed the length of the arms when drawing the second arc.

14. The length of the arms when he drew the first arc compared to second arc is different. The length of the arms when he drew the first arc is longer. When drawing the arms of a Perpendicular Bisector, the length of the arms should be same.

15. He shortened the length of the compass when drawing the other two arcs.

16. He did not extend the arms of the compass to A in the first place, and when he was drawing another arc, he shortened the length of the compass.

1. To describe more clearly - just make reference to the first attempt - he should not change the length when he drew the 2nd pair of arcs.

17. The length of the arms he used to draw the 2 arcs are different, and he did not stretch the arms to the whole length of the line

1. Ignore the part after the comma

2. The first part of the sentence is correct - the length between the pointed end and the pencil has changed when he drew the 2nd pair of arcs.
On the other hand, it is not necessary to stretch the arms to the whole length of the line. It will do the job as long as the arm is stretched beyond half of the length of the line.

18. He should keep the length of the compass the same when he drew the arcs from point B and point A, but because he shortened the length of the compass when drawing the arc from point A, his result will be inaccurate and he will not get a perpendicular bisector, which is a line that separates another line equally by half.

1. Correct and clear.

19. The length of the compass when he drew the two arcs were different.

1. Yes. You are right.

20. He should have used the length of the compass which he used to draw the first pair of arcs to draw the second pair of arcs. He should not have shortened the compass after drawing the first pair of arcs.

21. The length of the compass arms that he used to draw the first arc wasn't equal to the one he used to draw the second arc.

22. The length of the first one was not the same as length of the second time he drew with the compass so that was why his line was not perpendicular to the line.

1. If you try what he does, you would notice that he will still get a PERPENDICULAR line.
However, by changing the length of the arc, it is NO LONGER a BISECTOR.

23. He should not have shortened the length of the compass when he drew

1. Need to be more precise. When you said he should not have shortened the length of the compass, which step are you referring to? And can you elaborate why the length should not change?

24. The length between the compass arms should have been the same both times he drew the arcs.

1. To be more precise, it would be referring to the 2nd set of arcs. Can you elaborate why the length (actually it's the radius) of the arcs should be the same?

25. For the second arc drawn, he changed the length between the arms of the compass as compared to the length between the arms of the compass used for the drawing of the first arc. Also, for the drawing of the second arc, he did not stretch the arms of the compass a bit longer than half of the line.