 This video is called parallel perpendicular or neither. What we're going to do is look at equations of lines like y equals 2x plus 4 and y equals negative 1 half x plus 4. Equations of lines like this and decide are the lines parallel? Will they never intersect? Are they perpendicular? Will they intersect at a 90 degree angle or neither? Which means they'll cross. At some point they'll intersect but not at a 90 degree angle. It's important to remember that parallel lines have the same slope. And perpendicular lines have what's called opposite reciprocal slope. Remember opposite reciprocal, for example, if we had a line with the slope of two thirds, its opposite reciprocal slope would be negative three over two. The reciprocal is when the two thirds becomes three over two, the numerator and denominator switch places. The opposite part means that one of the slopes has to be positive, the other has to be negative. So another quick example, if you had a slope of negative seven, the line with an opposite reciprocal slope would be a positive one over seven. Because remember negative seven is really seven over one. So let's look. If you've got a problem like this asking if the lines are parallel, perpendicular or neither, you could find the slopes of both lines. The slopes are the numbers that are in front of the x, or you could think of it as multiplying the x. So in my first line, I have a slope of two. And in my second line, I have a slope of negative one half. Well clearly, the slopes are not the same. So we know that the lines are not going to be parallel. So the question is, are they perpendicular or neither? So ask, are they opposite reciprocals? Well, I have a positive two and a negative one half. One is positive and one is negative. And two and one half are reciprocals of each other. So we can say that those lines are definitely perpendicular. Let's try the second example. The second example has y equals one half x plus two. So we've got a slope of one half. And then down here in y equals one half x minus two, we also have a slope of one half. Since the slopes are the same, we know the lines are parallel.