Final answer:
Non-vertical parallel lines have equal slopes, while slopes of non-vertical, non-horizontal perpendicular lines are negative reciprocals of each other. Vertical lines are defined as having an infinite slope, thus they must be non-vertical to consider their slopes.
Step-by-step explanation:
When considering the slopes of lines on a graph with perpendicular axes, like the x-axis and the y-axis, we can draw several conclusions about parallel and perpendicular lines. If two non-vertical lines are parallel, their slopes are equal, because the tilt of the lines with respect to the horizontal axis is the same. For example, if we have two equations of lines in the form y = mx + b, where m represents the slope and b represents the y-intercept, parallel lines would have the same value of m.
If two lines are perpendicular and neither line is parallel to an axis, then their slopes are negative reciprocals of one another. This holds true because the product of the slopes of two perpendicular lines is -1. For example, if the slope of one line is a, the slope of the line perpendicular to it will be -1/a.
It is necessary for the lines to be non-vertical because a vertical line does not have a well-defined slope; its slope is considered infinite. Therefore, the definition of perpendicular lines in terms of slope only applies to non-vertical lines.