Final answer:
The slope can be used to determine if lines are parallel or perpendicular by comparing slopes; parallel lines have equal slopes, while perpendicular lines have slopes that are negative reciprocals of each other (product of slopes is -1).
Step-by-step explanation:
In mathematics, specifically in the study of geometry and linear graphs, the slope is an essential concept for determining the relationship between two lines. If two lines are parallel, the slope of one line is equal to the slope of the other line. To determine if an angle is 90 degrees, which means the lines are perpendicular, we look for the slopes of the lines. If the product of their slopes is -1, then the lines are perpendicular. Therefore, the correct answer to the question on how we can use slope to tell that opposite sides are parallel and that an angle is 90 degrees is A: Slope of opposite sides is equal; slope of a perpendicular line is -1.