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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 360^{o}.

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

180^{o}.

**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 90^{o}.

**Classification of angles**

Angles are classified according to their size.

We say that

- An angle with size α is
**acute**if 0^{o}< α < 90^{o}, - An angle with size α is
**obtuse**if 90^{0}< α < 180^{o}, - An angle with size α is
**reflex**if 180^{0}< α < 360^{o}

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