## Tuesday, 27 June 2017

### Geometrical Construction: Spot the Error (3)

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

1. The distance that he extended the compass' arms until was wrong as he was supposed to extend it to point B. The first arc was also redundant.

1. He should also start drawing at point B.

2. Watch the clip on how to construct the angle bisector.
In fact, the first step in the video was correct. It was the subsequent steps (drawing arcs from point B onwards) were incorrect.

2. He should not start drawing from point A and C but draw that the points of the first arc

3. He should draw the arcs starting from the arms not at point C and A

1. What do you mean "starting from the arms"?
Are you referring to "arcs" instead?

4. He should start drawing the last 2 arcs from the intersection points from the first arc.

1. The last part of the sentence could be clearer.
I think you are trying to say "from the points where the arcs intersects with AB and CB"?

5. He should extend the arms to point B from point C/A

1. Watch the clip (to construct the angle bisector) again - take note of the arcs and where the pointed end of the compass are placed:

6. He should not have drawn from point A and C but from the line that he had drawn earlier.

1. The first part of the sentence is correct.
The 2nd part, are you trying to say that he should have used the intersection points of the arcs and AB and CB respectively?

7. He should not have made the arcs from point A and C, but instead from the intersections from point B.

1. The first part of the sentence is correct.
What do you mean by "from the intersections from B"?

8. He should draw the arcs from the points intersected by the first arc but he drew the arcs from point A and C.

9. He should not have drawn the second arc from points A and C, but from the points of the first arc.

10. He should draw the arcs from the points which is not from points A and C but the first marked point.

1. The 2nd part of the sentence could be clearer by saying "... from the intersection points between the arcs and the AB and CB respectively"

11. He should have drawn from the first arc instead of a and c

1. Correct. It would be clearer if we say "... drawn from the intersection points between the first arc and the intersection points with AB and CB respectively"

12. When drawing the second and third arc, he should place the Compass Point at the intersection of the first arc and line AB and not at point A. He should also do likewise for line BC.

1. Correct and clear.

13. After drawing the first arc, he should continue drawing the second and third arcs from the first arc.

1. Correct. The 2nd part of the sentence could be clearer if we say, "... from the intersection points between the arc and BC and AB"

14. He should have placed his pointed end of his compass on the points where lines BC and BA intersected the arcs after drawing the first arc. From there, he should draw two arcs and then, draw the bisector through the intersection point of the two arcs and reach point B.

1. Correct and clear.

15. He should have drawn from his first arc instead of from point A and C.

1. To be clearer, it should be, "from the intersection points of the first arc with AB and CB"

16. He should extend his compass arm all the way to point b for both sides when drawing the two arcs at both sides.

1. Before extending the compass, has he placed the pointed end of the compass at the correct position before he attempted to draw the 2nd pair of arc?

17. For the second arc, he should place the compass at the points of the first arc.

1. You are correct. He should place the pointed end of the compass at where the first arc cuts the 'arms'.

18. For the third and fourth arc, he should have placed his compass point on the points where lines BC and BA intersected the first arc.

1. Very clear description. Good.

19. He should not be drawing the arcs from points A and C but instead from the point of the arc he first constructed.

1. You have described correctly what he should not have done.
The 2nd part of the description could be clearer when describing where he should place the pointed end of the compass at - the points where the first arc cuts the 'arms' (or you can say, AB and CB).

20. He should not have drawn the arcs A and C but the points of the first arc

1. The description needs to be more precise.
It's more like he should not have drawn the arc by placing the pointed end of the compass at points A and C. Instead he should have place the pointed end of the compass at the points where the first arc cuts the "arms" (AB and CB) to draw the two arcs.

21. He should have drawn the arcs from the point of the first arc he constructed and not at points A and C.

1. The first part could be clearer when you say, "the point where the first arc cuts the lines AB and CB".

22. He was correct at the first step of the video. However, he should have drawn the arcs from the intersection point of the first arc and lines AB and BC.