Ah, Pythagoras....

cos(c/r) = cos(a/r) * cos(b/r) I assume you mean (that's for a spherical triangle of course).

No, the question has many answers, only one of which is sqr(h) = sqr(a) + sqr(b).

Here are a few:

A right pain in the arse (13 year old kid).

The relationship between the real and imaginary components of a complex number.

One of the greatest developments in the history of Greek mathematics.

The theorem on p212 of your text book.

etc. etc.