21.1.3 What a Direct-Proportion Function Is, and What k Means
Core ideaThink of the k (k≠0) in y=kx as the fixed stride — how much y climbs every time x goes up by 1.
Module goal. Building on the coordinate plane and the idea of a function, meet the simplest family of functions — direct-proportion functions — and lock in the core picture: a graph that's a straight line through the origin.