1. Starter Questions B 5 A C 12
27o 1 1

2. To investigate the special angle in a semi-circle.
Angles in a Semi Circle
Learning Intention
To investigate the special angle in a semi-circle.

3. Angles in a Semi Circle Tool-kit required 1. Protractor 2. Pencil
3. Ruler 3

4. Angles in a Semi Circle 1. Using your pencil trace round
the protractor so that you have
semi-circle.
2. Mark the centre of the semi-circle.
You should have
something like this. 4

5. Angles in a Semi Circle x x x x x x Mark three points
Outside the circle x x x x x
2. On the circumference
3. Inside the circle

6. Angles in a Semi Circle Log your results in a table.
For each of the points
Form a triangle by drawing a
line from each end of the
diameter to the point.
Measure the angle at the
various points. x x x
Log your results in a table.
Inside
Circumference
Outside 6

7. Angles in a Semi Circle x Online Demo x x < 90o = 90o > 90o
Inside
Circumference
Outside
< 90o
= 90o
> 90o 7

8. Angles in a Semi-Circle
KeyPoint for Angles in a Semi-circle B A C
A triangle ABC within a semicircle
with base the length of the diameter
will ALWAYS be right angled at P on the circumference.
Remember - Angles in any triangle sum to 180o 8

9. Angles in a Semi-Circle
Example 1 : Sketch diagram and find all
the missing angles.
20o
Hints
43o
Look for right angle triangles
Remember !
Angles in any triangle
sum to 180o
47o
70o 9 9

10. Angles in a Semi-Circle
Example 2 : Sketch the diagram.
(a) Right down two right angle triangles
(a) Calculate all missing angles. D C
60o E
25o A B 10

11. Example of Good Work
Exercise
Number
Date
2 columns

12. Angles in a Semi-Circle
Now try Ex 2
Ch12 (page 138)
Sketch shapes