## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$...\underline{\text{their slopes are equal}}$ and $\underline{\text{their y-intercepts}}$ are unequal; $...\underline{\text{their slopes have a product of -1}}$.
The lines are parallel if their slopes are equal and the y-intercepts are unequal; the lines are perpendicular if their slopes have a product of $-1$.