The level varies, it can go from low level high school geometry, to higher levels. it is formulated a bit differently from regular trig though. so it might not be an approach taught widely in high school.
the use of such trig is the same as regular trig. only applied differently, with different notation and perhaps in different contexts. see Rational trigonometry - Wikipedia, the free encyclopedia
(I know it's a late reply, but someone reported this post and i wanted to answer it)