Finding congruent triangles - Math, Class 9 Video Lecture - Class 9 | Best Video for Class 9

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.