Parallel lines have the same slope, perpendicular lines have negative reciprocal slopes.
Practice makes perfect
Before we think about how you can recognize when lines are parallel or perpendicular, let's look at a graph with an example of each.
In this example, the red and blue lines are parallel and the green line is perpendicular to both of them. Visually, we see that lines are parallel because they will never intersect — they will continue on forever without ever crossing paths. Algebraically, we know that lines are parallel because they have the same slope. Let's take a look at the equations of the lines with the slopes highlighted.
y= 2x-2
y= 2x+1
Similarly, we can tell that lines are perpendicular when looking at them because when they intersect, it is at a perfect 90^(∘) angle. Algebraically, we can know that lines are perpendicular because their slopes are negative reciprocals.
y= 2x-2
y= -1/2x+2
