04 - Graphing Parabolas - Vertex and Axis of Symmetry - Free Educational videos for Students in K-12 | Lumos Learning

04 - Graphing Parabolas - Vertex and Axis of Symmetry - Free Educational videos for Students in k-12


04 - Graphing Parabolas - Vertex and Axis of Symmetry - By Math and Science



Transcript
00:00 Hello . Welcome back . I'm Jason with math and
00:02 science dot com . Today we're going to jump into
00:05 the concept of graphing parabolas . Specifically , we're going
00:08 to talk about the vertex of the parabola and the
00:10 axis of symmetry . I want you to have an
00:12 idea of the overall roadmap of where we're going this
00:14 lesson . We'll talk about the vertex in the axis
00:16 of symmetry . In the next few lessons , we'll
00:18 start working on how to shift these parabolas around the
00:22 xy plane . In other words , how to write
00:24 down the equation of a parabola and instead of it
00:26 being centered in the center of the xy plane ,
00:29 how it can be moved left and moved right and
00:31 so on . But before we get to that point
00:33 we have to understand some ideas and concepts in in
00:35 this case we're gonna be talking about the vertex in
00:37 the axis of symmetry . So what I need you
00:40 to do is to understand really burn it in your
00:44 mind . The basic parabola . Uh the reason I'm
00:51 talking about the basic problem is because in your mind
00:53 you need to have a general idea or or a
00:56 very um I say a general idea , but what
00:57 I mean is a , you have to burn this
00:59 image in your mind of what a parabola looks like
01:02 and where it is centered . I'm gonna call that
01:04 the basic parabola . The reason is because when we
01:06 start shifting the parabola around , then all we're gonna
01:09 do is take the original equation of the basic parabola
01:13 and change it very slightly in order to move it
01:15 around . So we have to have an idea of
01:17 what the basic one looks like . So you've already
01:20 encountered this , we've talked about it many times before
01:22 . The basic problem is very simple . F of
01:24 X is equal to x square . This is the
01:26 most basic parabola that you can get . And if
01:29 you don't want to think about functions , you can
01:30 write it instead of F of X . You can
01:32 say that Y is equal to X squared . Usually
01:34 when you start algebra you look at it in terms
01:37 of why is equal to X squared . And then
01:39 later on we understand the concept of a function and
01:42 so you replace the why with ffx . But functionally
01:44 these two things , these two representations are saying ,
01:47 the same thing . What they're saying is that we
01:49 stick numbers into this side of the equation and we
01:52 square them . And then the result gets applied to
01:55 . In this case it's a variable why ? In
01:56 this case it's a notation which is a little more
01:59 clear saying where that that this function is a function
02:01 of X because X is what we're changing . And
02:04 then the results kinda get spit out there and we
02:07 talked about the idea of what a function is in
02:08 the past . So uh we're not gonna do this
02:12 for every single problem but for this one , because
02:14 the basic problem is so important , I want to
02:16 write down a few points so we have X as
02:18 an input and then we have , I'm gonna use
02:20 the Y notation . Uh why is able to X
02:23 squared ? So we'll just make a quick little table
02:25 and I know that we've actually done this before but
02:27 I just want to do it here because we're going
02:29 to graphic uh as best we can and then we're
02:31 going to play around with it and and kind of
02:34 like so in the case of that we need to
02:36 have an idea of the basic problem in our minds
02:38 . So we need to pick some point . So
02:39 let's go from negative three , negative two , negative
02:41 10123 Of course you could go all the way to
02:45 negative five or negative 10 or positive five or whatever
02:48 . But in this case I'm just gonna stick to
02:50 negative three to positive three . So when we take
02:52 a negative three and we square it negative three times
02:55 negative three just gives us a nine negative two .
02:58 We square it . We get a positive four .
03:00 Same thing here , we get a positive 10 squared
03:02 is a 01 squared is a 12 squared is 43
03:07 squared is nine . So right away before we even
03:09 graph anything , you can already see some symmetry in
03:12 the in the shape of the problem . The center
03:15 of the thing is basically here , if we go
03:17 to positive values of X like this , then we
03:20 get larger and larger outputs which is the Y values
03:24 . But if we go to the negative values of
03:26 X , we get the exact same outputs as as
03:30 in the other side , the one matches with the
03:31 one and so on . So I can kind of
03:33 like just kind of like make it super obvious .
03:36 This one goes with this one , this one goes
03:38 with this one and the nine goes with this one
03:41 . And so we say that the graph of the
03:43 parietal is symmetric and we'll see it a little more
03:45 clearly when we draw it . But basically whether you
03:48 go to negative X values or positive X values ,
03:51 the outputs of the function give you exactly the same
03:54 thing . So it's like a mirror image , right
03:57 ? Um So let's go ahead and draw this graph
03:59 . It doesn't need to be perfect . I'm not
04:00 gonna pull out the crazy graph paper or anything like
04:04 that . We're just going to uh in general right
04:07 down what we need to burn the image in our
04:10 mind of what this problem is . So here we
04:11 have X . And here we have I can put
04:13 F of X . Here . I'll just put why
04:15 Since we're doing everything in terms of why . And
04:17 then we need to have some kind of tick marks
04:19 . Right ? So again , I went from in
04:21 my graph here negative three to positive three . So
04:23 let me go 123 So this is three . This
04:27 is to this is one , this is zero .
04:30 This is negative one , negative two negative three .
04:32 So I'll put negative three negative two negative one .
04:34 Right ? And then I when I graft this guy
04:37 , I ended up getting a large value of nine
04:39 . So I'm gonna go ahead and put nine Tech
04:40 marks 123456789 I could go on of course , but
04:46 I don't have any points that are beyond that place
04:50 . So let's go and plot this thing . We
04:52 have X comma Y which is zero comma zero .
04:54 Which means we have a point right here we have
04:57 one common one , which means we have one comma
04:59 one . Right here we have two comma four ,
05:03 which means we have two comma 1234 So that one
05:06 is actually somewhere right around there and then we have
05:09 three common nine , which means we have three .
05:11 And if I've counted correctly and we stand in front
05:13 to try to get it to look as good as
05:15 I can get it , it's not perfect . But
05:17 let's make it put a little bit higher Somewhere right
05:20 around here . So 123456789 is where that point is
05:25 . Now let's go the other direction negative one comma
05:27 one means negative one comma one is right here ,
05:30 negative two comma four means negative two , comma four
05:33 is right here and negative three comma nine means let
05:37 me stand in front so I can try to line
05:38 it up as good as I can . That's something
05:41 like that . It's not exactly right . So you
05:43 see this is not a straight line . I mean
05:45 if you try to draw a straight line through these
05:47 two points right here , then you don't hit this
05:49 one and you definitely don't hit this one so you
05:51 can see it's a curved kind of thing . So
05:53 I'm gonna do my best to sketch it but just
05:55 keep in mind that it's not going to be perfect
05:57 . So let's go down here through these two points
05:59 . We bend over , we go down here ,
06:02 we go up through these points like this of course
06:06 I could go off to the computer . But the
06:07 point is when you're on your , when you're on
06:09 your paper , when you're on your exams , you're
06:10 not gonna have a computer , right ? So of
06:12 course I could show you a computer image and I
06:13 have shown you already computer images of what a problem
06:16 it really looks like . It's a beautiful curve that
06:18 goes down like a smiley face kind of and then
06:20 it goes up for the for the one that opens
06:22 upward anyway . But for this , I want you
06:25 to burn this image in your mind . This is
06:27 what we call the basic parabola . It just is
06:30 F of X is equal to X squared , or
06:32 why is equal to X squared . And we plot
06:34 the points like this to show you the general shape
06:37 of the thing . But just keep in mind between
06:39 all of these points , Their infinite points in between
06:41 all of these points which I could of course fill
06:43 out in the table there that would trace out the
06:45 shape perfectly . And of course I could go on
06:47 and on forever writing points down . But the purpose
06:50 of this is not to be exact and it's not
06:53 to put a million points on the purpose of it
06:55 is so that you can see the basic shape .
06:56 What can we learn from this ? Right ? This
06:59 Parabola has a lowest point here , Right . And
07:03 this point here is called has a special name .
07:06 It's called the vertex . Mhm . So any time
07:09 when I say or a book says or a teacher
07:12 says , the vertex of the Parabola is located at
07:15 blah blah blah . All it means is that's the
07:17 lowest point of the parabola . Okay , of course
07:20 , I've drawn a problem that opens up like this
07:23 . So the lowest point is the vertex and this
07:25 vertex is located at zero comma zero . Now ,
07:28 later on , we're going to grab this parabola and
07:31 we're gonna move it all around . So I can
07:32 have the problem over here or I can have the
07:34 problem over here , I can have the problem over
07:36 here . The vertex of the problem will just be
07:39 the point at the lowest part of that parabola .
07:42 If it opens up like this , we'll see in
07:44 a minute . The problem can actually go upside down
07:46 , in which case the vertex will be the tippy
07:48 top . So it's either going to be the maximum
07:50 value of the problem or the minimum value of the
07:52 problem like it is here . But either way it's
07:54 called a vertex and it has a coordinate point in
07:57 this case zero comma zero . So for the basic
07:59 problem , the vertex is located at zero comma zero
08:02 . And as I've already hinted , we can of
08:05 course have parable is john all over this plane and
08:08 they can open up or they can open down ,
08:10 let's say how to problem that went and opened upside
08:12 down like this . In this case the vertex would
08:15 be whatever the maximum value was here . We're gonna
08:18 call that the vertex Uh here as well and I
08:21 give it some coordinate points or whatever . I'll do
08:24 some more detailed example later . But this coordinate might
08:26 be like 5:06 or something . They're So the vertex
08:30 is basically the maximum value . Now , the other
08:33 thing that I want to talk to you about is
08:35 what we call the axis of symmetry . It has
08:38 a very complicated sounding name , but it's actually very
08:40 , very simple . Okay . This thing , another
08:43 way to say it instead of calling it an axis
08:45 of symmetry is you can think of it as its
08:47 mirror image . Right ? This piece of paper is
08:49 a rectangle , right ? This piece of paper is
08:52 symmetric , it has a symmetry to it , right
08:54 ? Because why I can take this piece of paper
08:57 and I can fold it in half . And so
08:59 this piece paper has an axis of symmetry along the
09:02 long direction . If I could draw a dotted line
09:04 like through this piece of paper I have an axis
09:07 of symmetry of this thing because I can fold it
09:08 into mirror image . Also if I turn the piece
09:10 of paper sideways , I have an axis of symmetry
09:13 going like this because I can fold it across now
09:16 . Of course not every line is an axis of
09:19 symmetry . For instance if I try to draw an
09:21 axis of symmetry through the corners like that , it
09:24 doesn't quite work because if I try to fold it
09:26 from corner to corner , you see it doesn't really
09:28 line up and the thing has to line up exactly
09:31 for it to be a mirror image or an axis
09:33 of symmetry . So this parable has a very special
09:35 axis of symmetry . This one does as well .
09:37 And you can see that that axis of symmetry is
09:40 a line like this that goes vertically and cut goes
09:46 right through the vertex . It always has to go
09:48 through the vertex which is the center and this is
09:50 what we call the axis of symmetry . What it
09:57 means is it's the dotted line that I can fold
09:59 that terrible on itself and have it perfect . Have
10:02 it perfectly lined up . There's only one access that
10:04 works and it goes right through the vertex . The
10:07 axis of symmetry of this problem is the y axis
10:09 right here because the thing is centered there , I
10:11 can fold this thing on itself . Now . Before
10:14 we close the lesson out , I've introduced the concept
10:16 of what a vertex is . I've introduced the concept
10:19 of what an axis of symmetry is . Now .
10:20 I just want to write down and sketch a couple
10:22 of quick examples to show you numerically what these axes
10:25 are because in your problems , your your problem might
10:28 say here's a parabola , tell me what the vertex
10:30 is and what the axis of symmetry is and you'll
10:32 have to know how to do that . So let's
10:35 go over here and do that . Um What if
10:37 I had a parabola that looks like this ? I'll
10:39 go ahead and draw the axis , draw the problem
10:41 . We'll talk about what it is . Um Obviously
10:45 let's take a look at this problem . It's going
10:47 to be that nice standard , beautiful parable of shape
10:49 like this . Of course , This is X .
10:51 And this is why Okay , this is kind of
10:54 a gimme because we already drew it on the board
10:56 here . But what is the vertex of this problem
10:58 ? The vertex or the location of the vertex is
11:02 just the lowest point of the problem . And it's
11:03 right here at the origin . So the vertex just
11:06 as we said before is zero comma zero . The
11:08 axis of symmetry . I'm gonna write that as axis
11:12 of symmetry . You have to express that as a
11:15 line because an axis is a line , right ?
11:19 But the line that is the kind of the mirror
11:21 image center point here is the line right here at
11:25 X is equal to zero . Remember vertical lines in
11:28 algebra , when you plot vertical lines , they always
11:31 have the same form X equals something . Right ?
11:34 If I have a line at X equals five ,
11:37 that means I have a vertical line over here .
11:39 X is equal to five because the X values of
11:41 every point on this line over here , is that
11:43 five units of X ? Right ? In this case
11:45 it's X is equal to zero because the line is
11:47 going right through X is equal to zero . We
11:49 talked about vertical lines in the past , you should
11:51 know that , but I'm just refreshing your memory .
11:53 So the vertex is a point . The axis of
11:56 symmetry is a line . So this is not just
11:58 an X value . This is a line of points
12:01 that defines this guy . Now , let's take this
12:04 basic parabola and let's move it around a little bit
12:07 . Let's go over here and draw another problem which
12:12 is over here . Let's draw it over here like
12:17 this . So it looks exactly the same . It's
12:19 just shifted over . This is one , this is
12:23 two . This is three . All right . So
12:26 what would be the vertex in the axis of symmetry
12:28 here ? The vertex what would be the vertex ?
12:32 Well , that's the lowest point . The point here
12:34 . I've just shifted it from this point over here
12:36 . The point over here is located at two comma
12:38 zero . X is to y is zero . So
12:41 the vertex is now at a different location than the
12:43 vertex before . And the axis of symmetry . Yeah
12:49 . Is what ? It has to be a vertical
12:51 line , right ? It has to be a vertical
12:52 line that bisects or cuts this parable in half .
12:56 It has to go right through the vertex . So
12:58 the access of cemetery is X is equal to two
13:00 because it's a vertical line located at X is equal
13:03 to to all the set of points that go through
13:05 there . That's the axis of symmetry . So ,
13:09 here , we just basically took this basic parabola and
13:11 we just shifted it to the right and we can
13:13 very easily see how to handle the vertex and so
13:15 on . Now let's take and shift this Parabola in
13:18 a different location entirely . Right ? So instead of
13:21 shifting it over here , let's draw the parabola .
13:24 Let's draw the problem way up here in space ,
13:27 something like this . So it's not on any of
13:29 these access . What would the vertex in the axis
13:31 of symmetry be ? In that case ? Um Well
13:35 , you could say Uh 1 , 2 , 3
13:40 , one . Yeah , Kind of set up 1
13:44 , 2 . That's what I'm trying to go for
13:45 here like this . If I had drawn it like
13:48 this , then I could kind of died a line
13:51 over here and down the line over here and this
13:53 is X is equal to three and y is equal
13:55 to two . Right , then what would the vertex
13:58 B ? The vertex It's just the location of this
14:01 minimum 0.3 comma two . Basically you just read off
14:05 the coordinates of where the lowest part of that parabola
14:07 is . And then what would the axis of symmetry
14:09 ? B . Yes . Uh huh . It has
14:14 to be the vertical dotted line that cuts this thing
14:18 in half and it goes right down . You can
14:20 see it goes right through three . Right ? So
14:22 this axis of symmetry is X is equal to three
14:25 . So you can kind of see the pattern here
14:27 . The axis of symmetry always goes through the first
14:30 part of the vertex because that's the X coordinate of
14:33 where the vertex is , X equals zero . Goes
14:35 through this point X is equal to to the line
14:38 X is equal to , goes through the point to
14:41 the vertical line , X is equal to three goes
14:43 through this so they all match . So the axis
14:45 of symmetry is always going to be the first number
14:47 in the coordinate of the vertex . Alright now ,
14:51 so far all of these parables except for this one
14:55 that I drew have opened up and we're going to
14:58 find out an easy way to figure out if a
15:00 parabola opens up or opens down later . But for
15:03 now let's focus on what would the vertex in the
15:06 axis of symmetry look like for a problem that opens
15:09 kind of upside down . So instead of a smiley
15:12 face , like all of these problems , what if
15:13 it opens as a frowny face it goes upside down
15:16 . So if the problem goes upside down you might
15:18 have something like this . So let's take a look
15:20 at a quick sketch of one of these guys and
15:23 by the way it's not rocket science , it's not
15:25 gonna be any harder . Right ? So let's say
15:27 the problem instead of opening up , which is the
15:29 standard problems shape , It actually opens and comes down
15:33 like this . What is the vertex in the axis
15:35 of symmetry here ? It's not a big surprise .
15:37 The vertex is located at the highest point of this
15:41 guy , which is right at the origin and the
15:44 axis of symmetry is that x is equal to zero
15:47 . Same thing . It's just the first coordinate here
15:49 , because the line that bisects a is X is
15:51 equal to zero here . This is the X axis
15:53 , this is the y axis , so X is
15:55 equal to zero here . It goes right through there
15:57 . That's the vertex in the axis of symmetry .
16:00 And then we'll do one more before we close it
16:01 down . What if we had a problem over here
16:05 that let's try to be a little more precise ,
16:09 let's say this is negative one , this is this
16:11 is 123 And let's say that our vertex is gonna
16:15 end up being there . So I kind of gave
16:16 it away a little bit . But the point is
16:18 is that this Parabola goes something like this like this
16:26 , It opens upside down . What's the vertex in
16:28 the axis of symmetry ? What you can see that
16:30 the vertex is the is the kind of where these
16:32 kind of X and Y values there , the coordinates
16:35 of this top point here . So the vertex is
16:39 negative one comma three , that's X comma Y .
16:42 And the axis is going to be this line ,
16:45 whatever this line is , that by sexist thing which
16:47 has to be X is equal to negative one .
16:49 Right here , it's the one that goes through there
16:52 . So this lesson was all about introducing concepts ,
16:55 we're going to be taking these parabolas that we've been
16:57 learning about and we've kind of talked about problems being
17:00 quadratic functions , right ? And we have graft some
17:02 of them of course we already know how to graph
17:04 things . But what we have not done is done
17:07 terminology . You have to know what the vertex is
17:09 because why later on down the road we're going to
17:11 be writing these problems . Are these quadratic equations in
17:15 terms of their vertex . In other words , we're
17:17 gonna , you know when we talked about equations of
17:19 lines , you know , we learned three different ways
17:22 to write the equation of a line or at least
17:23 a couple of different ways . We learned the Mx
17:26 plus B . And then we learned the point slope
17:28 form and we learned a couple of different ways to
17:30 write the equations of lines . Well , there's more
17:32 than one way to write the equation of a parabola
17:35 . And so we're gonna learn about those . But
17:37 in order for us to get there , we have
17:39 to understand what the vertex and the axis of symmetry
17:41 of a problem is . So I want , you
17:43 know , mostly burn this in your mind , burn
17:46 the shape of the problem in where the lowest point
17:48 is for the basic problem , X squared which is
17:51 vertex is at 00 and X axis of symmetry being
17:53 X is equal to zero as a vertical line .
17:57 And then we did some other accommodations . We moved
17:59 it around and look at where the vertex moved .
18:01 Because in the next few lessons , we're gonna start
18:03 shifting these problems , we're gonna write equations that move
18:06 these problems . And I didn't show you what the
18:08 equation of this problem is that moves it over here
18:10 or the equation that moves it over here , we're
18:12 gonna write those equations down . It's gonna be much
18:14 easier if you understand the concept , the concepts in
18:17 this lesson . So make sure you understand this .
18:19 Follow me on to the next lesson will start shifting
18:21 parabolas around in the xy plane .
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