Two perpendicular lines, neither of which is vertical, must have slopes that are negative reciprocals of each other. In other words, if the slope of one line is m, the slope of the perpendicular line must be -1/m.
Two lines are perpendicular if and only if the product of their slopes is -1. Given that neither line is vertical, we can determine that the slopes must be negative reciprocals of each other. In other words, if the slope of one line is m, then the slope of the other line must be -1/m.
