Glossary

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a word class that may modify a verb (for example, 'beautifully' in 'She sings beautifully'), an adjective (for example 'really' in 'He is really interesting') or another adverb (for example 'very' in 'She walks very slowly'). In English many adverbs have an -ly ending.

An angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.

The size of an angle

Imagine that the ray OB is rotated about the point O until it lies along OA. The amount of turning is called the size of the angle AOB.

A revolution is the amount of turning required to rotate a ray about its endpoint until it falls back onto itself. The size of 1 revolution is 360o.

A straight angle is the angle formed by taking a ray and its opposite ray. A straight angle is half of a revolution, and so has size equal to
180o.

Right angle

Let AOB be a line, and let OX be a ray making equal angles with the ray OA and the ray OB. Then the equal angles ∠AOX and ∠BOX are called right angles.

A right angle is half of a straight angle, and so is equal to 90o.

Classification of angles

Angles are classified according to their size.

We say that

  • An angle with size α is acute if 0o < α < 90o,
  • An angle with size α is obtuse if 900 < α < 180o,
  • An angle with size α is reflex if 1800 < α < 360o

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