## 1.5 Trigonometric Functions Background Knowledge

#### Degrees and Radians

- All these while we are familiar with degrees. We know that a straight line is 180^{\circ}, and for a full circle it is 360^{\circ}.
- There is another unit to represent degrees, it is known as radians.
- The short form of radians can be written in several ways. Take 1 radian for example. The most formal way would be written as1^{c}. It can be written as 1\ \mathrm{rad} as before, or the most common way, 1 (without any units).
- The term ‘radian’ actually originated when we wanted to find a neat expression for the angle when the corresponding arc length of a circle with 1 unit radius, for this angle, is also 1 unit. Using degrees, the number will not look nice, hence a new unit called radians are invented.
- Radians are useful as it can be treated as numbers (without units).
- Radians are defined as positive for angles moving anticlockwise. For the diagram at the right, that is a positive angle (1 rad). It is similar to how we define 90^{\circ} and -90^{\circ}.
- The conversion from radian to degrees is: