With triangles and rules about them.
You can use trigonometry such as cos and tan
Anyone who knows more trigonometry than you and is able to convey that information effectively. A person with loads of qualifications does not necessarily make a teacher.
plane trigonometry spherical trigonometry
The main kinds are plane trigonometry and solid trigonometry. The latter will include trigonometry in hyper-spaces.
Plane trigonometry is trigonometry carried out in (on) a plane. This could be contrasted with spherical trigonometry, which is trigonometry carried out on the surface of a sphere. Certainly there are some other more complex forms of trig.
Trigonometry is applied in construction and building, as trigonometry measures right angled triangles.
Hipparchus is the father of trigonometry.
0.8660 is a shortened form of ' Sqrt(3) / 2 '. It refers to Sin(60 degrees) = opposite(sqrt(3) / Hypotenuse(2). Similarly the Complementary Sine ( Cosine/Cos) / Cos(30 degrees) = sqrt(3) / 2 A more accurate valuer for sqrt(3)/2 is 0.8669254038.... ) NB It is NOT the exact value, because sqrt(3 is an irrational number.
TRIGONOMETRY
A line in geometry has length (and location) but no width or depth. It's not itself a trigonometrical "being" but join 3 straight lines to make a Triangle, and Hey Presto! - you have Trigonometry!
Trigonometry based on a unit circle and radians and trigonometry based on a right triangle.
trigonometry is very imr