1. What an Angle Really Is
An angle is formed when two rays start from the same point. This common starting point is called the vertex of the angle, and the rays are called the arms of the angle.
To name an angle, we write three letters like \( \angle ABC \), where point \( B \) is the vertex. The order of letters shows how the angle is formed.
A
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\ (Angle at B)
B
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CThis simple idea helps us measure and classify angles by their size.
2. Classification of Angles by Measure
Angles are classified based on how wide they open. The size of an angle is measured in degrees (\( ^\circ \)). Below are the main types of angles, each with examples to help you visualise them.
2.1. Acute Angle
An acute angle is an angle that is less than \( 90^\circ \). It opens only a little and looks sharp or narrow. We can imagine the hands of a clock showing 2:00 — the smaller angle between the hour and minute hands is acute.
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\) (Less than 90°)
\2.2. Right Angle
A right angle measures exactly \( 90^\circ \). It forms an L-shape. Corners of a book, a square tile or a door frame all form right angles.
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----2.3. Obtuse Angle
An obtuse angle is an angle that is greater than \( 90^\circ \) but less than \( 180^\circ \). It opens wider than a right angle but does not form a straight line.
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\) (Wide angle)
\2.4. Straight Angle
A straight angle measures exactly \( 180^\circ \). It looks like a straight line. You can think of it as a half rotation around a point.
------------ (Straight line)
A2.5. Reflex Angle
A reflex angle is an angle that is greater than \( 180^\circ \) but less than \( 360^\circ \). It goes around the vertex more than a straight angle but does not complete a full turn.
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(Bigger opening)
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3. Naming and Reading Angles
We name an angle using three letters such as \( \angle ABC \), where:
- \( B \) is the vertex.
- \( BA \) and \( BC \) are the arms.
The vertex letter is always in the middle. This helps us avoid confusion when several angles meet at the same point. Sometimes only the vertex letter (like \( \angle B \)) is used, but only when the angle is clear in the diagram.
4. Angles Around Us
Different types of angles appear naturally in our surroundings. A few examples include:
- Acute: the sharp angle made by a partially opened book.
- Right: the corner of a classroom blackboard.
- Obtuse: the angle formed by the hands of a clock at around 9:30.
- Straight: a completely flat ruler placed on a desk.
- Reflex: the large outer angle in a fan's rotating blades.
Noticing these in daily life helps build a strong understanding of angle sizes and classifications.