This section has all been about calculating trigonometric functions and solving trigonometric equations. Trigonometry is a very practical branch of mathematics (if you're into that sort of thing).
It shows up in architecture, engineering, geography, astronomy, digital imaging, and a host of other fields. It can be used to calculate distances like the heights of mountains or how far away the stars in the sky are.
The cyclic, repeated nature of trig functions means that they are useful for studying different types of waves in nature: not just in the ocean, but the behavior of light, sound, and electricity as well. Another thing that repeats: pendulums. Yeah, you can make some cool stuff happen with trigonometry.