Trigonometry; the arch nemesis of high schools students. If you have any difficult in understanding Trigonometry and it's functions, here is an example:

About myself, when I was a student in university when I arrived 20 minutes late (which happened frequently enough, not because of oversleeping, because I would walk the route on foot) I would have to come in from the door of the classroom that by wonderful chance had one and only one door and one and only one very scrutinizing professor, I would try to tangentially slip through the door while his back was turned as he was writing on the blackboard and...

...and he, turning abruptly would apprehend me with a cotangential, sharp glare. 

My fate was a secant function at pi over two, unfortunate and undefined.