In plane geometry, an

**angle**is the figure formed by two rays, called the sides of the

**angle**, sharing a common endpoint, called the vertex of the

**angle**.

**Angles**formed by two rays lie in a plane, but this plane does not have to be a Euclidean plane.

Angles formed by the intersection of two curves in a plane are defined as the angle determined by the tangent rays at the point of intersection.

### Types of Angles

1.

**Acute Angle**: - Angle which is

**less than**

**90**

**° (θ < 90°)**

2.

**Right Angle**:- Angle which is

**equal to 90**

**° (θ = 90°)**

3.

**Obtuse Angle**:- Angle which is

**greather than 90**

**° (θ > 90°).**

4.

**Straight Angle**:- Angle which is

**equaal to 180**

**° (θ = 180°).**

5.

**Reflex Angle**:- Angle which is

**greater than 180**

**° but less than 360° (90° < θ < 360°)**

6.

**Complete Angle**:- Angle which is

**equal to**

**360° (θ=360°).**

### Positive and Negative Angles

**When measuring from a line:**

a positive angle goes

**counterclockwise**(opposite direction that clocks go)

a negative angle goes

**clockwise**

### Label Angles

**There are two main ways to label angles:**

1. give the angle a name, usually a lower-case letter like

**a**or

**b**, or sometimes a Greek letter like α (alpha) or

**θ**(theta)

2. or by the three letters on the shape that define the angle, with the middle letter being where the angle actually is (its vertex).

Example angle "

**a**" is "

**BAC**", and angle "

**θ**" is "

**BCD**"