I have not seen him "since" he left the house
* is that since a preposition, or a conjunction?

Preposition

Explanation:

thinking about things and referring to books instead of acting on superstition and gossip.

. a function is a relation in which each input has only one output. in the relation , y is a function of x, because for each input x (1, 2, 3, or 0), there is only one output y. x is not a function of y, because the input y = 3 has multiple outputs: x = 1 and x = 2.