Angles

 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"