Trigonometric functions

### 2.21.1 Trigonometric functions

- `sin` is the sine function,
- `cos` is the cosine function,
- `tan` is the tangent function (`tan(x)= sin(x)/cos(x)`),
- `cot` is the cotangent function (`cot(x)= cos(x)/sin(x)`),
- `sec` is the secant function (`sec(x)= 1/cos(x)`),
- `csc` is the cosecant function (`csc(x) = 1/sin(x)`),
- `asin` or `arcsin`, `acos` or `arccos`, `atan` or `arctan`, `acot`, `asec`, `acsc` are the inverse trigonometric functions. The latter are defined by:
  1. `asec(x) = acos(1/x)`,
  2. `acsc(x) = asin(1/x)`,
  3. `acot(x) = atan(1/x)`.