132
Drawing an angle of a given size
Textbook: Part 4, lesson 132, pp. 75–77
GoalDrawing an angle of a given size: the steps with a protractor; drawing an angle as a part of a right or straight angle.
New words
drawing an angle · burchak yasashray · nurmark · belgipart · qism
Explanation
Steps for drawing an angle: 1) draw a ray with a ruler: one side of the angle; its start is the vertex. 2) Put the protractor's centre on the start of the ray with 0° along the ray. 3) Mark a dot at the needed degree. 4) Draw the second ray from the vertex through the dot. An angle can also be given as a part of a straight or right angle: 1/4 of a straight angle = 180° : 4 = 45°.
Worked examples
4/9 of a straight angle: 180 : 9 × 4 = 80°.
3/5 of a right angle: 90 : 5 × 3 = 54°.
Class activity
“Draw the angle”: the teacher calls a size (40°, 75°, 120°), children draw it with a protractor and a partner measures to check.
Practice
1
How many degrees is 1/4 of a straight angle?
45
2
How many degrees is 7/12 of a straight angle?
105
3
How many degrees is 3/5 of a right angle?
54
4
How many degrees is the angle adjacent to a 50° angle?
130
5
Angle ABC is 35°. Ray BD splits it and angle ABD is 20°. How many degrees is angle DBC?
15