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## High school geometry

### Course: High school geometry > Unit 6

Lesson 4: Parallel & perpendicular lines on the coordinate plane# Classifying figures with coordinates

CCSS.Math:

Use coordinates to determine the slopes of sides of a figure in order to classify it. Created by Sal Khan.

## Want to join the conversation?

- How exactly would taking the opposite reciprocal confirm that it is a 90 degree angle?(5 votes)
- Sal explains why if two lines have opposite reciprocal slopes, they are perpendicular in a different video (https://www.khanacademy.org/math/geometry/hs-geo-analytic-geometry/hs-geo-parallel-perpendicular-eq/v/proof-that-perpendicular-lines-have-negative-reciprocal-slope)(3 votes)

- how am I supposed to find the answer when he uses a graph every time, but none of the problems give you a graph to solve it on?(2 votes)
- Why do we have to learn geometry？(1 vote)
- It has applications in all areas of mathematics and in all hard-sciences, also it's occasionally very beautiful (see penrose tiling and the golden ratio)(2 votes)

- At the start of the video I noticed that the points have related x and y values that go in some sort of shape that goes all the way around without crossing the other sides. did anybody else notice that? And does that also mean that any shape like that makes a rectangle?(1 vote)
- There is a pattern, which has to do with the fact that the rectangle is actually a square.

Going clockwise around the square,

it just so happens to be that the sum of the 𝑥-coordinate of one corner and the 𝑦-coordinate of the next corner is constant,

and the difference between the 𝑦-coordinate of one corner and the 𝑥-coordinate of the next corner is also constant.

This is true for all squares.(1 vote)

- how do people get this(1 vote)

## Video transcript

- [Instructor] We're told
that parallelogram ABCD has the following vertices, and they give us the coordinates
of the different vertices. And they say is parallelogram
ABCD a rectangle, and why? So pause this video, and
try to think about this on your own before we
work through it together. All right, now let's
work through it together. So in general, if you know that something
is already a parallelogram and you wanted to determine
whether it's a rectangle, it's really a question of
whether the adjacent sides intersect at a right angle. So for example, a parallelogram might
look something like this. What we know about a parallelogram is that the opposite sides are parallel, so this side is parallel to that side and that this side is
parallel to this side. And all rectangles are parallelograms, but not all parallelograms are rectangles. In order for a parallelogram
to be a rectangle, these sides need to
intersect at right angles. And clearly, the way I drew this one, it doesn't look like that. But let's see if we can
figure that out based on the coordinates that
they have given us. And to help us visualize, let me just put some coordinates. Let me draw some axes here. So that's my x-axis, and then this is my y-axis. Let's see, the coordinates, let's see, we have twos, fours, sixes, so let me actually count by, and eights, let me count by twos here. So we have two, four, six, and eight, and then we have negative
two, negative four, negative six, negative eight. We have two, four, six, and eight, and then we'd have negative
two, negative four, negative six, and negative eight. So each hash mark is another two. I'm counting by twos here. And so let's plot these points, and I'll do it in a different
color, so we can keep track. So A is negative six comma negative four. So negative two, negative
four, negative six, and then negative four
would go right over here. That is point A. Then we have point B, which
is negative two comma six. So negative two comma six, so that's going to go
up two, four, and six. So that is point B right over there. Then we have point C, which
is at eight comma two. So eight comma two right over there, that is point C. And then last but not
least, we have point D, which is at four comma negative eight, four comma negative eight right over there, point D. And so our quadrilateral, or we actually know it's a
parallelogram, looks like this. So you have segment AB like that. You have segment BC that looks like that. Segment CD looks like this. And segment AD looks like this. And we know already that
it's a parallelogram, so we know that segment AB
is parallel to segment DC and segment BC is parallel to segment AD. But what we really need
to do is figure out whether they are
intersecting at right angles. And to do that using the
coordinates to figure that out, we have to figure out the slopes of these different line segments. And so let's figure out
first the slope of AB. So the slope of segment AB is going to be equal to our change in y over change in x. So our change in y is going
to be six minus negative four, six minus negative four over negative two minus negative six, negative two minus negative six. And so this is going to
be equal to six plus four, which is 10, over negative two minus negative six. That's the same thing as
negative two plus six. So that's going to be over four, which is the same thing as 5/2. All right, that's interesting. What is the slope of segment BC? The slope of segment BC is going to be equal to, once again, change in y over change in x. Our y-coordinates' change
in y is two minus six, two minus six over eight minus negative two, eight minus negative two, which is equal to negative four over, and then eight minus negative
two is the same thing as eight plus two over 10, which is the same thing as negative 2/5. Now in other videos in your algebra class, you might have learned
that the slopes of lines that intersect at right
angles or the slopes of lines that form a right angle at
their point of intersection, that they are going to be
the opposite reciprocals. And you can actually see
that right over here. These are opposite reciprocals. If you take the reciprocal of
this top slope, you'd get 2/5. And then you take the opposite of it or, in this case, the negative of it, you are going to get negative 2/5. So these are perpendicular lines. So this lets us know that AB is perpendicular, segment AB is perpendicular to segment BC. So we know that this is the case. And we could keep on doing that. But in a parallelogram, if one set of segments
intersect at a right angle, all of them are going to
intersect at a right angle. And we could show that more
rigorously in other places, but this is enough evidence for me to know that this is indeed
going to be a rectangle. If you want, you could
continue to do this analysis, and you will see that
this is perpendicular, this is perpendicular, and
that is perpendicular as well. But let's see which of
these choices match up to what we just deduced. So choice A says yes, and yes would be that it is a rectangle because AB is equal. So the length of segment AB is equal to the length of segment AD, and the length of segment BC is equal to the length of segment CD. So that might be true. I haven't validated it. But just because this is true and because we do know that
ABCD is a parallelogram, that wouldn't let me know that we are actually
dealing with a rectangle. For example, you can have a parallelogram where even all the sides are congruent. So you could have a parallelogram
that looks like this. And obviously, if all of
the sides are congruent, you're dealing with a rhombus, but a rhombus is still
not necessarily going to be a rectangle. And so I would rule this top one out. This second choice says yes, and it says because BC
is perpendicular to AB. Yeah, we saw that by
seeing that their slopes are the opposite
reciprocals of each other. And of course, we know that
ABCD is a parallelogram. So I am liking this choice. And these other ones claim
that this is not a rectangle, but we already deduced
that it is a rectangle. So we could rule these out as well.