80
Angles. Kinds of angles
Textbook: Part 3, lesson 75, pp. 8–9
GoalAngle, its vertex and sides; acute, right and obtuse angles.
New words
angle · burchakvertex · burchak uchiright angle · to‘g‘ri burchakset square (right-angle ruler) · goniya
Explanation
An angle is the part between two rays that start at one point. The point is the vertex and the rays are the sides. Folding a sheet in half twice makes a right angle; we check it with a set square. A narrower angle is acute and a wider angle is obtuse.
Worked examples
∠BAC or ∠A: A is the vertex, AB and AC are the sides.
At 3:00 the clock hands make a right angle; scissors opened a little make an acute angle, opened wide an obtuse angle.
Class activity
“Angle hunt”: children find right angles in the classroom (window, notebook, door) and check them with a set square, then look for something showing an acute or obtuse angle.
Practice
1
What is an angle narrower than a right angle called?
An acute angle
2
What is an angle wider than a right angle called?
An obtuse angle
3
An exhibition has 118 pictures. 48 are in colour pencil, 14 fewer in ordinary pencil, and the rest in chalk. How many are in chalk?
36
4
Solve: x + 318 = 700. x = ?
382