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## Geometry (all content)

### Course: Geometry (all content) > Unit 2

Lesson 8: Sal's old angle videos# Angles of parallel lines 2

Angles of parallel lines examples. Created by Sal Khan.

## Want to join the conversation?

- Aren't obtuse angles greater than 90°? I'm a bit confused, since the obtuse angle is labeled 60° and the acute angle is labeled 120°.(55 votes)
- Not drawn to scale, darling.(5 votes)

- what are alternate interior, alternate exterior and consecutive angles?(12 votes)
- Since you can't draw with your answer, think about an H with the middle line going out past the two parallel lines. the alternate interior are the angles on opposite sides (such as the top left and the bottom right or the top right and bottom left corners, NOT the top and bottom left or top and bottom right) of the middle line that are inside the two parallel lines. Alternate exterior are just like alternate interior except that they are the angles outside of the two parallel lines. Consecutive angles are the angles that touch each other, the top and bottom corners, and they are inside the parallel lines, although some teachers say they can also be outside the parallel lines.(12 votes)

- Who else agrees that the first problem needs to be corrected ASAP? That is a very obvious mistake when he calls the larger angle 60 degrees, while the angle half its size is 120 degrees! No offense intended, but it needs corrected so no one else is confused by this, agreed?(10 votes)
- Ok. So, the only reason it seems wrong, is because the drawing is not "fit to scale." --> this means that the drawing might not look like he says it is.... Meaning the 60 degrees and the 120 degrees might not look like they "should" be. I do get what you mean though.(4 votes)

- at1:54can you explain what in the world he's talking?(3 votes)
- He's saying that both of those angles are equal to 180 degrees. We already know one of them is 60, so to figure out what the other one is you just subtract 60 from 180. And that will leave you with 120. So that "mystery angle" is 120.(4 votes)

- Why was the the first angle identified as 60º?

That seems very confusing when that angle couldn't possible <90º.

I'm sure you weren't trying to be exacting. But couldn't you at least pick an example angle that is possible, like 110º, 120º, 130º, etc?(4 votes) - How come the acute angle is an obtuse angle and the obtuse angle is the acute angle? he made a mistake!(2 votes)
- Never assume that diagrams are drawn to scale. Often test questions will draw the diagram "wrong" on purpose, you can only use the numbers you are given to solve these problems. Don't worry about if it looks right or not.(4 votes)

- Ummm 60 should be the smaller angle since anything under 90 is a acute angle and the other angle is clearly obtuse.(2 votes)
- that is true but ignore that and listen to what he is saying(2 votes)

- what is the sum of interior angles on the same side of transversal between two parallel lines(2 votes)
- Any two angles like that are a value of 180.(2 votes)

- Why did he label the acute angles as 120 degrees and the obtuse angles 60 degrees in the beginning? Shouldn't it be the other way around?(2 votes)
- yes an obtuse angle is and angle over 90 degrees and 60 degrees is an angle under 90 which is called and acute angle.(1 vote)

- At0:33, isn't an obtuse angle w/ a measure of 60 degrees an impossibility?(2 votes)

## Video transcript

Let's do a couple of examples
dealing with angles between parallel lines and
transversals. So let's say that these two
lines are a parallel, so I can a label them as being parallel. That tells us that they will
never intersect; that they're sitting in the same plane. And let's say I have a
transversal right here, which is just a line that will
intersect both of those parallel lines, and I were to
tell you that this angle right there is 60 degrees and then I
were to ask you what is this angle right over there? You might say, oh, that's
very difficult; that's on a different line. But you just have to remember,
and the one thing I always remember, is that corresponding
angles are always equivalent. And so if you look at this
angle up here on this top line where the transversal
intersects the top line, what is the corresponding angle to
where the transversal intersects this bottom line? Well this is kind of the bottom
right angle; you could see that there's one, two,
three, four angles. So this is on the bottom
and kind of to the right a little bit. Or maybe you could kind of view
it as the southeast angle if we're thinking in
directions that way. And so the corresponding
angle is right over here. And they're going
to be equivalent. So this right here
is 60 degrees. Now if this angle is 60
degrees, what is the question mark angle? Well the question mark angle--
let's call it x --the question mark angle plus the 60 degree
angle, they go halfway around the circle. They are supplementary; They
will add up to 180 degrees. So we could write x plus
60 degrees is equal to 180 degrees. And if you subtract 60 from
both sides of this equation you get x is equal to 120 degrees. And you could keep going. You could actually figure out
every angle formed between the transversals and
the parallel lines. If this is 120 degrees,
then the angle opposite to it is also 120 degrees. If this angle is 60 degrees,
then this one right here is also 60 degrees. If this is 60, then its
opposite angle is 60 degrees. And then you could either say
that, hey, this has to be supplementary to either this
60 degree or this 60 degree. Or you could say that this
angle corresponds to this 120 degrees, so it is also 120, and
make the same exact argument. This angle is the same
as this angle, so it is also 120 degrees. Let's do another one. Let's say I have two lines. So that's one line. Let me do that in purple and
let me do the other line in a different shade of purple. Let me darken that other
one a little bit more. So you have that purple
line and the other one that's another line. That's blue or
something like that. And then I have a line that
intersects both of them; we draw that a little
bit straighter. And let's say that this angle
right here is 50 degrees. And let's say that I were also
to tell you that this angle right here is 120 degrees. Now the question I want to
ask here is, are these two lines parallel? Is this magenta line and
this blue line parallel? So the way to think about is
what would have happened if they were parallel? If they were parallel, then
this and this would be corresponding angles, and so
then this would be 50 degrees. This would have to
be 50 degrees. We don't know, so maybe I
should put a little asterisk there to say, we're not sure
whether that's 50 degrees. Maybe put a question mark. This would be 50 degrees if
they were parallel, but this and this would have to be
supplementary; they would have to add up to 180 degrees. Actually, regardless of whether
the lines are parallel, if I just take any line and I have
something intersecting, if this angle is 50 and whatever this
angle would be, they would have to add up to 180 degrees. But we see right here that this
will not add up to 180 degrees. 50 plus 120 adds up to 170. So these lines aren't parallel. Another way you could have
thought about it-- I guess this would have maybe been a more
exact way to think about it --is if this is 120 degrees,
this angle right here has to be supplementary to that; it
has to add up to 180. So this angle-- do it in this
screen --this angle right here has to be 60 degrees. Now this angle corresponds
to that angle, but they're not equal. The corresponding angles
are not equal, so these lines are not parallel.