Grade 4 (age 9-10)

Angles

An angle is how far two rays spread apart from a single point. You meet angles everywhere, like opening scissors or a door. With a protractor you measure the size in degrees: 90 is a right angle, smaller is acute, and larger is obtuse. Slide the angle open and watch its size change and how far the clock hand turns.

What is an angle?

📐 Two lines meet → an angle

①Open scissors → an angle forms

②Open them more → angle gets bigger

③Measure size in degrees (°)

④A protractor measures it precisely

Measure with a Protractor

Angle45°

💡 Types of Angles

①Acute: 0°-90° (sharp)

②Right: exactly 90° (L-shape, square corner)

③Obtuse: 90°-180° (wide)

④Straight: 180° (a line)

Angles in Daily Life

🔄 Angles and Rotation

①One full clock turn = 360°

②Half turn = 180° (straight)

③Quarter turn = 90° (right)

④Triangle's 3 angles sum = 180°

Summary

Right Angle

90° = right angle

Square corner, L-shape

Types of Angles

Acute < 90° < Obtuse < 180° (Straight)

<90° = acute, >90° = obtuse

One Turn

360° = one full rotation

Angle of one full clock turn

🎯 Key Points

①Angle = how far two lines spread

②Unit is degree (°)

③Right = 90°, straight = 180°

④Acute < Right < Obtuse < Straight

⑤Triangle angles always sum to 180°

Examples and Unit Test

Example 1

Is a 70° angle acute, right, or obtuse?

An acute angle is greater than 0° and less than 90°.

70° is less than 90°, so it is acute.

▸ Acute

Less than 90° = acute, exactly 90° = right, more than 90° = obtuse.

Example 2

Two angles of a triangle are 60° and 70°. What is the third angle?

The three angles of a triangle always add up to 180°.

180° − 60° − 70° = 50°, so the third angle is 50°.

▸ 50°

The three angles of a triangle always sum to 180°.

Unit Test

How many degrees is one full turn of a clock hand?

①90°

②180°

③270°

④360°

▸ ④ 360°

One full turn is one complete rotation.

A half turn is 180°, and a full turn is twice that: 360°.

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Estimation

Triangles