@Burgoo King:
Remember that not all high schools in the entire world teach the exact same topics in mathematics. I can't remember for sure, but I think my high school didn't officially teach it either (in the last one or two years of my high school we had a class where the teacher raced through the normal material and then had time left for "hobbying around"; teaching stuff (well) outside the regular curriculum to the kids who where curious about mathematics and wanted to know more).
On a related note, unless you are a teacher, you don't get to prescribe
how people proof something, any valid proof is, well, a valid proof. (The proof for the preceding statement: "Tautology, QED".)
@Moros:
Does this
http://9gag.com/gag/6599111 solve your piano question? :p