Introduction to angles (old) What an angle is. Angles in a circle. Complementary and supplementary angles.
Introduction to angles (old)
- Hello. In this series of presentations, I'm gonna try
- to teach you everything you need to know about triangles and angles and parallel lines
- and this is probably the highest-yield information that you could ever learn, especially in terms of the standardized tests.
- And then when we've learned all the rules we'll play something I call the Angle Game,
- which is essentially what the SAT makes you do over and over again.
- So let's start with some basics.You know what an angles is.
- Well actually maybe you don't know what an angle is.
- If I have two lines...
- and they intersect at some point,
- the angle is a measure of exactly how wide the intersection is between those two lines.
- So this is the angle. An angle is how wide those two lines open up.
- And they're measured either in degrees or radiants. And for the sake of most geometry classes we'll use degrees.
- When we start doing Trigonometry we'll use radiants.
- And you're probably familiar with this. Zero degrees would be two lines on top of each other...
- this if I were to just eyeball it looks like 45 degrees.
- If I had the lines even wider apart, like that, that's 90 degrees.
- And 90 degree lines are also called perpendicular, because
- they are, I feel like saying because they are perpendicular,
- but because one is going completely vertical while the other is going horizontal.
- Wow, it's actually amazingly difficult to find the exact right wording.
- But I think you get the idea. By definition, perpendicular lines are 90 degrees apart from each other.
- And you've seen this all the time in things like squares or rectangles.
- A rectangle is made up of a bunch of perpendicular lines, or lines at 90 degree angles.
- The way you draw a 90 degree angle is you draw a little box like that.
- That's the same thing as doing this.
- And you could even get wider angles. If you go above 90 degrees... this could be, I don't know, 135 degrees
- If you ever want to really measure the angles you could use a protractor.
- Then if you had it so wide that the two lines are actually almost forming a line...
- that's 180 degrees. And then you could keep going.
- If this angle is 135 degrees...
- There are 360 degrees in a circle. So this magenta angle would be 360 - 135 degrees
- that's 225 degrees.
- So you know degrees in a circle are 360 degrees, this is important to know.
- It's also important to know that if you go halfway around a circle,
- that's 180 degrees.
- Like if you viewed the pivot point as like,
- let's say, right here.
- I mean it looks like just one line and it really is.
- But that's 180 degrees.
- And then if you go quarter way around the circle,
- that's 90 degrees.
- All right?
- Hopefully you're getting a bit of an intuition
- for what an angle is.
- So now I will teach you a bunch of very useful
- rules for angles.
- Clear this.
- So let me redraw.
- So if I had a line like this.
- I like using the colors, just so I think it keeps you from
- getting completely bored.
- And it might not be completely intuitive what I'm doing, but
- let's add an angle like that.
- And so, let's just say-- you know, I'm not measuring these
- exactly-- let's say that this is 30 degrees.
- We know that if we go all the way around the circle, we know
- that that's 360 degrees.
- And that's a very ugly looking around the circle
- angle that I drew.
- So then we also know that this angle right
- here is 330 degrees.
- Because this angle plus this magenta angle is going to
- equal the whole circle.
- So this is equal to 330 degrees.
- So remember that.
- The angles in a circle-- or there are 360
- degrees in a circle.
- I don't know if you remember.
- You probably don't.
- This was probably before you were born.
- But there used to be a game called 720, and it was a
- skateboarding game-- it was a video game.
- And the 720 was essentially you were trying to jump
- your skateboard and spin around twice.
- And that's 720 degrees.
- If you go around a circle twice that's 720 degrees.
- If you just jump and spin around once, you
- went 360 degrees.
- So you've probably heard this in just popular culture.
- But anyway.
- So 360 degrees in a circle.
- And you could imagine half a circle is 180 degrees.
- So the other important thing to realize is, like we said, if
- we go halfway around the circle it's 180 degrees.
- But if we have two angles that add up to that-- so let's say.
- I don't know if these lines are thick enough for you to see.
- Let me draw something thicker.
- It doesn't look ideal, but you get the idea.
- So if we have this angle, let's call it x.
- And then this angle is y.
- What do we know about the relationship between x and y?
- Well, we know that the entire angle is half of a circle.
- So that's 180 degrees.
- That's 180 degrees, this entire angle.
- So what are angles x and y going to add up to?
- I'm trying to stay color consistent.
- x plus y are going to equal-- I'm color blind,
- I think-- 180 degrees.
- Or you could write y is equal to 180 minus x.
- Or x is equal to 180 minus y.
- But if x plus y are equal to 180 degrees-- and you can see
- that it makes sense that they do-- if you add the two angles
- you go halfway around a circle.
- Then that tells us that x and y are-- and this is a fancy word,
- and it's just good to commit this to memory-- they are
- supplementary angles.
- That's when you add to 180 degrees.
- Now what if we had this situation.
- Oh my God, that was horrible.
- Let's say I had this situation.
- Let's see.
- I draw two perpendicular lines.
- So this is going a quarter way around the circle.
- All right.
- Let's say this entire angle here-- I'm drawing it really
- big-- that's 90 degrees.
- They're perpendicular.
- And now if I had two angles within that.
- So now if I have two angles here-- so let's say that this
- is x and this is y-- what do x and y add up to?
- Well, x plus y is 90.
- And we can say that x and y are complementary.
- And it's important to not get confused between the two.
- Just remember complementary means two angles add up to 90
- degrees, supplementary means that two angles add
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
Have something that's not a question about this content?
This discussion area is not meant for answering homework questions.
Share a tip
When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
Thank the author
This is great, I finally understand quadratic functions!
Have something that's not a tip or thanks about this content?
This discussion area is not meant for answering homework questions.
At 2:33, Sal said "single bonds" but meant "covalent bonds."
For general discussions about Khan Academy, visit our Reddit discussion page.
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
- disrespectful or offensive
- an advertisement
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
- a tip or thanks in Questions
- a question in Tips & Thanks
- an answer that should be its own question