129
Measuring angles with a protractor
Textbook: Part 4, lesson 129, pp. 66–68
GoalMeasuring an angle with a protractor: placing it correctly, reading the scale, deciding the kind of angle.
New words
protractor · transportirscale · shkalavertex of the angle · burchak uchimeasuring · o‘lchash
Explanation
A protractor measures angles from 0° to 180°. 1) Put the centre of the protractor on the vertex of the angle. 2) Line up the 0° mark with one side of the angle. 3) Read the degree where the other side crosses the scale. If the protractor has two scales, read the one whose 0° is on the first side. Then decide the kind of angle from the result.
Worked examples
If the second side crosses 70°, the angle is 70°, acute. 104° is obtuse, 90° is right.
If two rays from one vertex cross the 40° and 130° marks counted from 0°, the angle between them is 130° − 40° = 90°.
Class activity
“Angle master”: each child measures angles of a notebook, table and door with a protractor, writes the measurement and the kind of angle, and a classmate checks.
Practice
1
The second side of an angle crosses 70° on the scale starting from 0°. How many degrees is the angle and what kind is it?
70°, acute
2
What kind of angle is 143°?
obtuse
3
Is an 86° angle acute, right or obtuse?
acute
4
Two rays from one vertex pass the 40° and 130° marks counted from 0°. How many degrees is the angle between them?
90
5
What is the largest angle in degrees that a protractor can measure?
180