131
Protractor: mixed practice
Textbook: Part 4, lesson 131, pp. 72–74
GoalMeasuring with a protractor: mixed practice – kinds of angles, sums and differences, angles of a quadrilateral, parts of a circle.
New words
mixed exercise · aralash mashqkind of angle · burchak turiangle of a sector · sektor burchagiturn · burilish
Explanation
In this lesson we combine measuring angles, naming their kinds, adding and subtracting. Remember: acute < 90°, right = 90°, obtuse > 90° and < 180°, straight = 180°; adjacent angles on a line add up to 180°; angles of a triangle add up to 180°; if we measure and add the four angles of a quadrilateral we get 360°; a full turn is 360°. A circle split into n equal sectors has a central angle of 360° : n in each.
Worked examples
If two angles of a triangle are 90° and 35°, the third is 180° − 90° − 35° = 55°.
A circle split into 10 equal sectors: 360° : 10 = 36°.
Class activity
“Angle relay”: teams take turns at the board to measure a given angle or say its kind; correct answers earn points.
Practice
1
How many of the angles 35°, 120°, 90° and 170° are obtuse?
2
2
Angles of 25° and 65° are placed side by side. How many degrees is the combined angle?
90
3
A triangle has angles 90° and 35°. How many degrees is the third angle?
55
4
A circle is split into 10 equal sectors. How many degrees is the central angle of each?
36
5
Three angles of a quadrilateral are 80°, 95° and 70°. How many degrees is the fourth angle?
115