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### Course: Geometry (all content) > Unit 11

Lesson 5: Theorems concerning quadrilateral properties# Proof: Opposite angles of a parallelogram

Sal proves that opposite angles of a parallelogram are congruent. Created by Sal Khan.

## Want to join the conversation?

- When comparing angles of equal measure in this video Sal uses a congruent symbol. Would it be equally acceptable to use a plain equals sign instead?

If not, when would you use a congruent symbol and when would you use an equals sign to compare angles?(16 votes)- No, it would not. If you were talking specifically about the measures of the angles, then you could, but when referring to the angle itself, you should use the congruence symbol.(6 votes)

- Is point D a right angle at0:53?(3 votes)
- Technically,
*point D*is not an angle :)

Since Sal is talking about parallelograms, the angle BDC (the angle defined by the line segments BD and CD) isn't necessarily a right angle, but it's not impossible for it to be a right angle.

You can't*assume*that BDC is a right angle, because if you did, you'd only prove that the opposite angles of a parallelogram are equal*if one of the angles is a right angle*(from which would follow that the other three angles are right angles too).

When proving things, you want to make as little assumptions as possible so that your proof applies to as many scenarios as possible.(11 votes)

- Math really is another language. And I am not fluent.

How does one name angles? It seems like it's random mostly, but I'm not sure.

Just wondering. :D(7 votes)- In all reality, you can name an angle anything, as long as it is a variable. But sometimes people name them as numbers, but the y have to be in order and they all have to in numbers. Hope this helped.(3 votes)

- Aren't opposite angles of a parallelogram corresponding angles or alternate interior angles?(5 votes)
- No they are not corresponding or alternate interior angles. They are just called opposite angles and they are equal to each other.(2 votes)

- Wait, but couldn't you also find the vertical angle of <BDC, because that would also correspond to <CAB?(3 votes)
- You are correct that they are congruent as vertical angles, but what purpose does it serve?(4 votes)

- What about a trapezoid?(3 votes)
- If 𝐴𝐵𝐶𝐷 is a trapezoid, where side 𝐴𝐵 is parallel to side 𝐶𝐷,

then angles 𝐴 and 𝐷 are supplementary, as are angles 𝐵 and 𝐶.(3 votes)

- why is the ~ place top on the =?(0 votes)
- That means that the two figures are congruent. Just ~ means similar. Hope I helped!(8 votes)

- What is the difference between
*Congruence*and*Equivalence*?(2 votes)`The fact is that they are not the same. Congruence is a relationship of shapes and sizes, such as segments, triangles, and geometrical figures, while equality is a relationship of sizes, such as lengths, widths, and heights. Congruence deals with objects while equality deals with numbers.`

(3 votes)

- does congruent mean identical in form, like a copy of a figure?(2 votes)
- Yes, it is often referred to as same shape, sane size.(3 votes)

- define congruent, does it mean equal(1 vote)
- In geometry two shapes are congruent if they have the same shape or size. So yes, congruent means "Equal in size or shape."(5 votes)

## Video transcript

What I want to do
in this video is prove that the opposite
angles of a parallelogram are congruent. So for example, we want to prove
that CAB is congruent to BDC, so that that angle is
equal to that angle, and that ABD, which
is this angle, is congruent to DCA, which
is this angle over here. And to do that, we
just have to realize that we have some
parallel lines, and we have some transversals. And the parallel lines
and the transversals actually switch roles. So let's just continue these
so it looks a little bit more like transversals
intersecting parallel lines. And really, you could
just pause it for yourself and try to prove it, because
it really just comes out of alternate interior angles
and corresponding angles of transversals
intersecting parallel lines. So let's say that this
angle right over here-- let me do it in a new
color since I've already used that yellow. So let's start right
here with angle BDC. And I'm just going
to mark this up here. Angle BDC, right over here-- it
is an alternate interior angle with this angle right over here. And actually, we could
extend this point over here. I could call this
point E, if I want. So I could say angle CDB
is congruent to angle EBD by alternate
interior angles. This is a transversal. These two lines are parallel. AB or AE is parallel to CD. Fair enough. Now, if we kind of change
our thinking a little bit and instead, we now view BD
and AC as the parallel lines and now view AB as
the transversal, then we see that
angle EBD is going to be congruent to
angle BAC, because they are corresponding angles. So angle EBD is going to
be congruent to angle BAC, or I could say CAB. They are corresponding angles. And so if this angle is
congruent to that angle and that angle is
congruent to that angle, then they are congruent
to each other. So angle-- let me make sure
I get this right-- CDB, or we could say BDC, is
congruent to angle CAB. So we've proven this first
part right over here. And then to prove that
these two are congruent, we use the exact same logic. So first of all, we view
this as a transversal. We view AC as a
transversal of AB and CD. And let me go here and let
me create another point here. Let me call this point
F right over here. So we know that
angle ACD is going to be congruent to
angle FAC because they are alternate interior angles. And then we change our
thinking a little bit. And we view AC and BD
as the parallel lines and AB as a transversal. And then angle FAC is going
to be congruent to angle ABD, because they're
corresponding angles. Angle F to angle ABD, and
they are corresponding angles. So in the first time, we viewed
this as the transversal, AC as a transversal of AB and
CD, which are parallel lines. Now AB is the transversal and BD
and AC are the parallel lines. And obviously, if this
is congruent to that, and that is congruent
to that, then these two have to be congruent
to each other. So we see that if we have
opposite angles are congruent-- or if we have a
parallelogram, then the opposite angles are
going to be congruent.