The length of what?
usually its used to find a missing angle or length of a right triangle. Of course there is more to trigonometry. any way you can use sine, cosine, and tangent, to fine the missing angle or length
You need to use trigonometry cosine (angle) = length/diagonal diagonal = length/cos(angle)
By using trigonometry or Pythagoras' theorem
In general, you measure it. In a few special cases you may be able to use geometry or trigonometry.
Obviously, the pyramids have a lot to do with triangles. To find the hight, length, width, volume, angle of ascent, and surface area of the pyramids, the egyptians had to use trigonometry.
it is used in real life situation to fin the height of thing and in math you use it to find the length of one of the sides.
Whenever an architect makes a line on his bleprint that is not straight or does not lie parallel to the x, y, or z axes, he must use trigonometry to find its length and and the angle it makes with other lines.
If it's a right angle triangle and an acute angle plus the length of a leg is given then use trigonometry to find the hypotenuse.
The question is to vague/general. Measure it? Find the positive square root of the area? Use the diagonal for trigonometry (45-45-90)?
I find it very interesting.
In trigonometry, when we look at right triangles, the cosine is the ratio of the length of the adjacent side to the length of the hypotenuse.
If there is no length for the hypotenuse you have to use the Pythagorean Theorem. If there are two sides missing and a reference angle you could use Trigonometry.