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## High school geometry

### Course: High school geometry > Unit 3

Lesson 3: Congruent triangles- Triangle congruence postulates/criteria
- Determining congruent triangles
- Calculating angle measures to verify congruence
- Determine congruent triangles
- Corresponding parts of congruent triangles are congruent
- Proving triangle congruence
- Prove triangle congruence
- Triangle congruence review

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# Determining congruent triangles

Sal uses the SSS, ASA, SAS, and AAS postulates to find congruent triangles. Created by Sal Khan.

## Want to join the conversation?

- How would triangles be congruent if you need to flip them around?(7 votes)
- Congruent means the same size and shape. It doesn't matter if they are mirror images of each other or turned around. If you could cut them out and put them on top of each other to show that they are the same size and shape, they are considered congruent. Rotations and flips don't matter.(59 votes)

- in ABC the 60 degree angle looks like a 90 degree angle, very confusing.... :=D(13 votes)
- Math teachers love to be ambiguous with the drawing but strict with it's given measurements. Always be careful, work with what is given, and never assume anything.(16 votes)

- Can you expand on what you mean by "flip it". I cut a piece of paper diagonally, marked the same angles as above, and it doesn't matter if I flip it, rotate it, or move it, I cant get the piece of paper to take on the same position as DEF.(7 votes)
- The triangles that Sal is drawing are not to scale.(19 votes)

- how are ABC and MNO equal? both of their 60 degrees are in different places(10 votes)
- Basically triangles are congruent when they have the same shape and size. So if you have two triangles and you can transform (for example by reflection) one of them into the other (while preserving the scale!), the two triangles are congruent.

If you flip/reflect MNO over NO it is the "same" as ABC, so these two triangles are congruent.(13 votes)

- does it matter if a triangle is congruent by any of SSS,AAS,ASA,SAS?(12 votes)
- Your question should be about two triangles. This is because by those shortcuts (SSS, AAS, ASA, SAS) two triangles may be congruent to each other if and only if they hold those properties true.

SSS: When all three sides are equal to each other on both triangles, the triangle is congruent

AAS: If two angles and a non-included (you can think of it as outside) side of one triangle are congruent to the corresponding parts of another triangle, then the triangles are congruent.

ASA: If two angles and the included side of one triangle are congruent to the corresponding parts of another triangle, then the triangles are congruent.

SAS: If any two angles and the included side are the same in both triangles, then the triangles are congruent.(5 votes)

- Is there a way that you can turn on subtitles?(5 votes)
- Yeah. There's this little button on the bottom of a video that says CC. That will turn on subtitles. Different languages may vary in the settings button as well. If you hover over a button it might tell you what it is too.(11 votes)

- why doesn't this dang thing ever mark it as done(6 votes)
- IDK. It happens to me though. When it does, I restart the video and wait for it to play about 5 seconds of the video. Then I pause it, drag the red dot to the beginning of the video, push play, and let the video finish. For some unknown reason, that usually marks it as done. I hope it works as well for you as it does for me.(9 votes)

- wouldn't some AAA triangles be congruent as well?(4 votes)
- We don't consider AAA triangles to not be congruent, we simply say they can't be proven congruent, but still can be(5 votes)

- what does congruent mean?(2 votes)
- Congruent means same shape and same size. So congruent has to do with comparing two figures, and equivalent means two expressions are equal. So to say two line segments are congruent relates to the measures of the two lines are equal.(9 votes)

- Why are AAA triangles not a thing but SSS are? If you can't determine the size with AAA, then how can you determine the angles in SSS?(3 votes)
- Two triangles that share the same AAA postulate would be
**similar**. But if all we know is the angles then we could just dilate (scale) the triangle which wouldn't change the angles between sides at all.

If we know that 2 triangles share the SSS postulate, then they are**congruent**. This means that they can be mapped onto each other using rigid transformations (translating, rotating, reflecting, not dilating).

There is only 1 such possible triangle with side lengths of A, B, and C. Note that that such triangle can be oriented differently, using rigid transformations, but it will 'always be the same triangle' in a manner of speaking.

Angles tell us the relationships between the opposite/adjacent side(s), which is what sine, cosine, and tangent are used for.

Hope this helps!

- Convenient Colleague(7 votes)

## Video transcript

What we have drawn over here
is five different triangles. And what I want to
do in this video is figure out which
of these triangles are congruent to which
other of these triangles. And to figure that
out, I'm just over here going to write our triangle
congruency postulate. So we know that
two triangles are congruent if all of their
sides are the same-- so side, side, side. We also know they are congruent
if we have a side and then an angle between the sides
and then another side that is congruent-- so
side, angle, side. If we reverse the
angles and the sides, we know that's also a
congruence postulate. So if we have an angle
and then another angle and then the side in
between them is congruent, then we also have two
congruent triangles. And then finally, if we
have an angle and then another angle and
then a side, then that is also-- any of these
imply congruency. So let's see our
congruent triangles. So let's see what we can
figure out right over here for these triangles. So right in this
triangle ABC over here, we're given this length 7,
then 60 degrees, and then 40 degrees. Or another way to
think about it, we're given an angle, an angle
and a side-- 40 degrees, then 60 degrees, then 7. And in order for something
to be congruent here, they would have to have an
angle, angle, side given-- at least, unless maybe
we have to figure it out some other way. But I'm guessing
for this problem, they'll just already
give us the angle. So they'll have to have an
angle, an angle, and side. And it can't just be any
angle, angle, and side. It has to be 40, 60, and 7, and
it has to be in the same order. It can't be 60 and
then 40 and then 7. If the 40-degree side
has-- if one of its sides has the length 7, then that
is not the same thing here. Here, the 60-degree
side has length 7. So let's see if any of
these other triangles have this kind of 40,
60 degrees, and then the 7 right over here. So this has the 40 degrees
and the 60 degrees, but the 7 is in between them. So this looks like
it might be congruent to some other triangle,
maybe closer to something like angle, side,
angle because they have an angle, side, angle. So it wouldn't be that one. This one looks interesting. This is also angle, side, angle. So maybe these are congruent,
but we'll check back on that. We're still focused on
this one right over here. And this one, we have a 60
degrees, then a 40 degrees, and a 7. This is tempting. We have an angle, an
angle, and a side, but the angles are
in a different order. Here it's 40, 60, 7. Here it's 60, 40, 7. So it's an angle,
an angle, and side, but the side is not on
the 60-degree angle. It's on the 40-degree
angle over here. So this doesn't
look right either. Here we have 40 degrees,
60 degrees, and then 7. So this is looking pretty good. We have this side
right over here is congruent to this
side right over here. Then you have your 60-degree
angle right over here. It might not be obvious,
because it's flipped, and they're drawn a
little bit different. But you should never assume
that just the drawing tells you what's going on. And then finally, you have
your 40-degree angle here, which is your
40-degree angle here. So we can say-- we can
write down-- and let me think of a good
place to do it. I'll write it right over here. We can write down that triangle
ABC is congruent to triangle-- and now we have to be very
careful with how we name this. We have to make
sure that we have the corresponding
vertices map up together. So for example, we started
this triangle at vertex A. So point A right
over here, that's where we have the
60-degree angle. That's the vertex of
the 60-degree angle. So the vertex of the 60-degree
angle over here is point N. So I'm going to go to N. And then we went from A to B. B
was the vertex that we did not have any angle for. And we could figure it out. If these two guys add
up to 100, then this is going to be the
80-degree angle. So over here, the
80-degree angle is going to be M, the one that
we don't have any label for. It's kind of the
other side-- it's the thing that shares the 7
length side right over here. So then we want to go to
N, then M-- sorry, NM-- and then finish up
the triangle in O. And I want to
really stress this, that we have to make sure we
get the order of these right because then we're referring
to-- we're not showing the corresponding
vertices in each triangle. Now we see vertex
A, or point A, maps to point N on this
congruent triangle. Vertex B maps to
point M. And so you can say, look, the length
of AB is congruent to NM. So it all matches up. And we can say
that these two are congruent by angle,
angle, side, by AAS. So we did this one, this
one right over here, is congruent to this
one right over there. And now let's look at
these two characters. So here we have an angle, 40
degrees, a side in between, and then another angle. So it looks like ASA is
going to be involved. We look at this one
right over here. We have 40 degrees, 40
degrees, 7, and then 60. You might say, wait, here are
the 40 degrees on the bottom. Then here it's on the top. But remember, things
can be congruent if you can flip them-- if
you could flip them, rotate them, shift them, whatever. So if you flip
this guy over, you will get this one over here. And that would not
have happened if you had flipped this one to
get this one over here. So you see these two by--
let me just make it clear-- you have this 60-degree angle
is congruent to this 60-degree angle. You have this side
of length 7 is congruent to this
side of length 7. And then you have
the 40-degree angle is congruent to this
40-degree angle. So once again,
these two characters are congruent to each other. And we can write-- I'll
write it right over here-- we can say triangle DEF is
congruent to triangle-- and here we have to
be careful again. D, point D, is the vertex
for the 60-degree side. So I'm going to start at H,
which is the vertex of the 60-- degree side over here-- is
congruent to triangle H. And then we went
from D to E. E is the vertex on the 40-degree
side, the other vertex that shares the 7 length
segment right over here. So we want to go
from H to G, HGI, and we know that from
angle, side, angle. And so that gives us that
that character right over there is congruent to this
character right over here. And then finally, we're left
with this poor, poor chap. And it looks like it is not
congruent to any of them. It is tempting to try to
match it up to this one, especially because the
angles here are on the bottom and you have the 7 side
over here-- angles here on the bottom and
the 7 side over here. But it doesn't match up,
because the order of the angles aren't the same. You don't have the same
corresponding angles. If you try to do this
little exercise where you map everything
to each other, you wouldn't be able to
do it right over here. And this over here-- it might
have been a trick question where maybe if you
did the math-- if this was like a 40 or a
60-degree angle, then maybe you could
have matched this to some of the other triangles
or maybe even some of them to each other. But this last angle, in all
of these cases-- 40 plus 60 is 100. This is going to be an
80-degree angle right over. They have to add up to 180. This is an 80-degree angle. If this ended up, by the math,
being a 40 or 60-degree angle, then it could have been a
little bit more interesting. There might have been
other congruent pairs. But this is an 80-degree
angle in every case. The other angle is 80 degrees. So this is just a lone--
unfortunately for him, he is not able to find
a congruent companion.