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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?

• 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)
• at can 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)
• 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)
• At , isn't an obtuse angle w/ a measure of 60 degrees an impossibility?
(2 votes)
• What if the two corresponding add up to a number above 180?
(2 votes)
• Pairs of corresponding angles need not be supplementary When a transversal intersects parallel lines, corresponding angles are congruent. That means they could both be, for example, 153 degrees -- in which case they would sum to 306 degrees.
(1 vote)
• If a line that is exactly straight what is its degrees?
(1 vote)
• If a line is exactly straight it adds up to exactly 180 degrees. No more no less.
(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.