8th Grade · Expressions & Equations
Constant of Proportionality
8.EE.B.5
Definition
The constant ratio k in a proportional relationship y equals k times x. It equals the unit rate and the slope of the graph.
Examples
- If 4 apples cost 12 dollars, k = 12/4 = 3 dollars per apple.
- On the graph, the line rises 3 for every 1 across — k is the slope.
Key rule
k is the unit rate, the slope, AND the value of y when x=1 — all the same number.
Memory hook
k = y/x, always.