Slope Is the Rate of Change
A linear function changes by the same amount for every unit step, so any two points on it give the same difference quotient -- the slope -- and a graph through the origin is a direct proportion whose slope is the unit rate. Shifting a line or reading a runner's speed off a graph uses that constant rate; a graph made of straight pieces, like an area that grows and then shrinks as a point moves, has its own constant slope on each piece and changes rate only where the pieces join.
grade 7–8 FunctionsExpressionsRatios Draw a DiagramAnalyze the Units
Progression (4)
g7 1. Read each runner's rate from one graph point RatiosNumber system
g8 2. Every difference quotient is the slope ExpressionsFunctions
g8 3. Trade a vertical shift for a horizontal one Functions
g8 4. Graph a moving point's area piece by piece FunctionsGeometry