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

$...\text{ are perpendicular if } a = -\frac{1}{2}$.
RECALL: (i) Non-vertical lines are parallel if they have equal slopes. (ii) Non-vertical and non-horizontal lines are perpendicular if their slopes have a product of −1 Thus, for the given lines to be perpendicular, the product of their slopes must be $-1$, For this to happen, the value of $a$ must be $-\frac{1}{2}$.