Conic Sections: Hyperbolas 3 Part 3 of the intro to hyperbolas
Conic Sections: Hyperbolas 3
⇐ Use this menu to view and help create subtitles for this video in many different languages. You'll probably want to hide YouTube's captions if using these subtitles.
- Let's see if we can tackle a slightly more difficult
- hyperbola graphing problem.
- Let's add the hyperbola.
- Make this up on the fly x minus 1 squared over 16 minus y plus
- 1 squared over 4 is equal to 1.
- So the first thing to recognize is that this is a hyperbola and
- we'll in a few videos, do a bunch of problems where the
- first point is just to identify what type of conic section we
- have and then the second step is actually graph
- the conic section.
- I already told you that we're going to be doing a hyperbola
- problem, so you know it's a hyperbola.
- But the way to recognize that is that you have this minus of
- the y squared term and then we actually have it shifted.
- The classic or the standard non-shifted form of a hyperbola
- or a hyperbola centered at 0 would look something like this.
- Especially if it has the same asymptotes just shifted, but
- centered at 0 it would look like this: x squared over 16
- minus y squared over 4 is equal to 1.
- And the difference between this hyperbola and this hyperbola
- the center of this hyperbola is at the point x is equal to
- 1 y is equal to minus 1.
- And the way to think about it is x equals 1 makes this whole
- term 0, and so that's why it's the center.
- And y equal to minus 1 makes this whole term 0.
- And on here, of course, the center is the origin.
- Center is 0, 0.
- So the easy way to graph this is to really graph this one,
- but you shift it so you use the center being 1 minus 1 instead
- of the center being 0, 0.
- So let's do that.
- So let's figure out the slope of the two asymptotes here and
- then we can shift those two slopes so that it's appropriate
- for this hyperbola right here.
- So if we go with this one, let's just solve for y.
- That's what I always like to do whenever I'm
- graphing a hyperbola.
- So we get minus y squared over 4.
- Subtracting x squared over 16 from both sides minus
- x squared over 16 plus 1.
- I'm working on this hyperbola right here, not this one,
- and then I'm going to just shift it later.
- And then let's say multiply both sides by minus 4 and you
- get y squared is equal to-- see the minus cancels out with that
- and then 4 over 16 is x squared over 4 minus 4 and so y is
- equal to plus or minus square root of x squared
- over 4 minus 4.
- And to figure out the asymptotes you just have to
- think about well what happens as x approaches positive
- or negative infinity.
- As x gets really positive or x gets really negative.
- And we've done this a bunch of times already.
- I think this is important.
- This is more important than just memorizing the formula,
- because it gives you an intuition of where those
- equations for the lines of the asymptote actually come from.
- Because these are what this graph or this equation or this
- function approaches as x approaches positive or
- negative infinity.
- As x approaches positive or negative infinity, what is y
- approximately equal to, in this case?
- Well once again, this term is going to dominate.
- This is just a 4 right here.
- You could imagine when x is like a trillion or a negative
- trillion, this is going to be huge number and this is going
- to be just like you know you almost view it like
- the ground off area.
- You take the square root of that and so this
- is going to dominate.
- So as you approach positive or negative infinity, y is going
- to be approximately equal to the square root, the positive
- and negative square root, of x squared over 4.
- So y would be approximately equal to positive or
- negative x over 2, or 1/2x.
- Let's do that.
- Let's draw our asymptotes.
- And remember, these are the asymptotes for this situation.
- But now of course, we're centered at 1 negative 1.
- So I'm going to draw two lines with these slopes, with
- positive 1/2 and negative 1/2 slopes, but they're going to be
- centered at this point.
- I just got rid of the shift just so I could figure out the
- asymptotes but of course this is the real thing that we're
- trying to graph, so let me do that.
- This is my y-axis this is my x-axis and the center of
- this is at 1 negative 1.
- So x is equal to 1 y is equal to minus 1.
- And then the slopes of the asymptotes were positive
- and negative 1/2.
- So let's do the positive 1/2.
- So that means for every 2 you run over, so if you go positive
- in the positive x direction 2, you move up 1.
- So you go to the right 2 and up 1.
- So that's the first one.
- Let me draw that asymptote.
- Looks something like that, and then we draw it from this
- point to that point.
- Got to have a steady hand.
- And then the other asymptote is going to have a
- minus 1/2 slope.
- Remember this is our center 1 minus 1, so if
- I go down 1 and over.
- So when I go over 2, I go down 1, so that will be right there,
- Let me draw that asymptote.
- And then just to continue it in the other direction I want
- to make the lines overlap.
- It's going to look something like that.
- So we've drawn our asymptotes for this function, and now we
- have to figure out if it's going to be a vertical
- hyperbola or a horizontal hyperbola.
- And the easy way to think about it is to try and make--
- and we can do it two ways.
- I mean if you just look at this equation right here.
- When you're taking the positive square root, we're always going
- to be slightly below the asymptote.
- The asymptote is this thing, but we're always going
- to be slightly below it.
- So that tells us that were always going to be slightly
- below the asymptote on the positive square root, and we're
- always going to be slightly above the asymptote on the
- negative square root.
- Because it's going to be little less, and it's negative.
- But I'll let you think about that.
- My intuition is that it's going to be there and there.
- It's more than intuition.
- I know that we're going to be a little bit less than the
- negative square root, but I'll do it the other way.
- I'll do it the way I did in the last video.
- So the other way to think about it is what happens
- when this term is 0?
- For this term to be 0, x has to be equal to 1.
- And does that ever happen?
- Can x be equal to 1?
- If x is equal to 1 here this term is 0.
- And then you have a situation where-- and then you have a
- minus y squared over 4 would have to equal 1, or this would
- have to be a negative number.
- So x could not be equal to 1.
- So y could be equal to negative 1.
- Let's try that out.
- If y is equal to negative 1, this term right
- here disappears.
- So when y is equal to negative 1, you're just left with-- x
- minus 1 squared over 16 is equal to 1.
- I just canceled out this term, because I'm saying what happens
- when y is equal to negative 1.
- You multiply both sides by 16.
- Let me do it over here.
- These get messy. x minus 1 squared is equal to 16.
- Take the square root of both sides.
- x minus 1 is equal to positive or negative 4.
- And so if x is equal to positive 4, if you add 1 to
- that x would be equal to 5.
- And then if x minus 1 would be minus 4 and you add 1 to that
- you will have x is equal to 3.
- So our 2 points or our 2 points closest to our center are the
- points 5 comma negative 1 and 3 comma negative 1.
- Let's plot those 2.
- So 5, 1 2 3 4 5, negative 1 and 3, negative 1.
- Is that right?
- No, minus 3, because x minus 1 could be minus 4.
- That's what happens when you skip steps.
- If you have the minus 4 situation, then x is
- equal to minus 3.
- You go 1 2 3 minus 3, minus 1.
- So those are both points on this hyperbola.
- And then our intuition was correct, or it was what I said.
- That-- the positive square root is always going to be slightly
- below the asymptote, so we get our curve.
- It's going to look something like this.
- It's going to get closer and closer, and then here it's
- going to get closer and closer in that direction.
- It keeps getting closer and closer to that asymptote.
- And here, it's going to keep getting closer and closer to
- the asymptote on that side and then on that side.
- And of course these asymptotes keep going
- on forever and forever.
- If you want you could try out some other points
- just to confirm.
- You could plot that point there, or that point there
- just to confirm that that's the case.
- The hard part really is just to identify the asymptotes and
- just to figure out do we sit on the left and the right, or do
- we sit on the top and the bottom.
- And then you're done.
- You can graph your hyperbola.
- See you in the next video.
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...
Have something that's not a tip or feedback about this content?
This discussion area is not meant for answering homework questions.
Discuss the site
For general discussions about Khan Academy, visit our Reddit discussion page.
Flag inappropriate posts
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 feedback in Questions
- a question in Tips & Feedback
- an answer that should be its own question
about the site