Unit 3
Straight-line relationships and the coordinate plane. Slope carries a rate with units, which is what makes linear models interpretable.
Lesson 3.1
Slope measures change in output per unit change in input. Use the same point order in numerator and denominator. Positive slope indicates increasing output; a horizontal line has slope zero. A vertical line has undefined slope because its change in x is zero.
Lesson 3.2
Slope-intercept form displays slope and vertical intercept directly. Point-slope form is useful when a point and slope are known. Standard form can reveal intercepts by setting one variable to zero. Converting between forms changes the representation, not the line.
Lesson 3.3
Distinct parallel nonvertical lines have equal slopes and different intercepts. Perpendicular nonvertical lines have slopes whose product is -1. Horizontal and vertical lines are perpendicular, a special case not covered by the product rule because a vertical slope is undefined.
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