AAHL 2.13.1 Rational functions (quadratic in denominator)
Learn AAHL 2.13.1 Rational functions (quadratic in denominator) in this free IB Mathematics Analysis and Approaches (AA) video lesson for AHL 2.13—Rational functions.
Learn AAHL 2.13.1 Rational functions (quadratic in denominator) in this free IB Mathematics Analysis and Approaches (AA) video lesson for AHL 2.13—Rational functions.
Learn AAHL 2.13.1 Rational functions (quadratic in denominator) in this free IB Mathematics Analysis and Approaches (AA) video lesson for AHL 2.13—Rational functions.
The video provides a detailed lesson on sketching rational functions that involve quadratics, focusing on two main types: one with a quadratic in the denominator and another with a quadratic in the numerator.
Key steps for sketching the graph include:
A sign diagram is used to analyze the function's behavior across different intervals, ultimately aiding in the accurate sketching of the graph, which features key points and asymptotes.
00:00Hi everyone. So I'm going
00:02to do two lessons on
00:03these rational functions with quadratic.
00:05So this is part one
00:06and I'll have to be
00:07part two. Basically, one of
00:10them you have your quadratic
00:11in the denominator and the
00:14other one you have your
00:14quadratic in the numerator and
00:16then the other function is
00:17just a linear. So it's
00:18like a linear function over
00:20a quadratic. So I've listed
00:25need to do in order
00:26to sketch this. So the
00:28S and it's just basically
00:29sketching it, but obviously once
00:31you sketch it, you can
00:32do all the other things
00:33like solve equations or inequalities
00:39or whatever you need to
00:40do. So this lesson is
00:42just going to focus on
00:43sketching this to kind of
00:44get an idea of what
00:45it looks like. So the
00:46first thing is the x
00:47intercepts. Let's do them step
00:48by step. So the first
00:49thing I'm going to do
00:50here, the first thing I'm
00:52going to do is find
00:53the x intercept. Now the
00:54x intercept
00:56happens when y equals zero.
00:58So this is zero. New
00:59narrator has to be zero.
01:01So literally, all I'm left
01:02with is x minus one
01:03equals zero, x equals one.
01:06So the x intercept is
01:09when x equals one. Second
01:11one. The y intercept with
01:14the y intercept happens when
01:15x equals zero. So x
01:17is zero, zero, zero, zero.
01:18I'm not sure what negative
01:19one over negative six, which
01:22is one over six. So
01:23the y intercept
01:24is y equals one over
01:27six, three. The horizontal asymptote.
01:32Okay, when the quadratic is
01:36in the bottom, the horizontal
01:38asymptote is always y equals
01:42zero. And the reason for
01:44that is the horizontal asymptote
01:46explained this in the previous
01:48rational functions. Listen,
01:52the horizontal lesson is basically
01:54what's happening when x is
01:55really really big or really
01:57really small. So are really
01:59really really large in the
02:00positive direction or the negative
02:02direction. When imagine x is
02:03a million then you're going
02:04to have a million over
02:06a million million which is
02:09basically one over a million
02:10which is very very very
02:11small. So because this the
02:14degree of this polynomial in
02:16the denominator is bigger than
02:17the degree of this linear
02:18function here as x
02:20gets really really large, y
02:22gets really really really small
02:24and it approaches zero. So
02:26you have your horizontal last
02:27note of y equals zero.
02:30For the vertical last note,
02:33so the vertical last note
02:35occur when the denominator equals
02:40zero. I explain that again
02:44in the other lesson, but
02:48it makes sense
02:48And because if you think
02:49as the denominator approaches zero,
02:53the function approaches infinity, because
02:55you're dividing by a really
02:56small number. But you can't
02:58actually divide by zero, so
02:59you get an asymptote. So
03:02this sounds when the denominator
03:04equals zero, x squared minus
03:06x minus six equals zero.
03:08Now we're going to have
03:08to solve this, and we're
03:09going to have two vertical
03:13asymptotes. So I'll just factorize
03:16this quickly.
03:16this is x minus 3x
03:20plus 2x equals 3 or
03:25x equals negative 2. So
03:27these are my two vertical
03:31asymptotes. And then finally what
03:34I'm going to do is
03:35assign diagram right now this
03:39may be your first time
03:41seeing this. So let me
03:44explain it but it's quite
03:47it's quite neat or interesting
03:49or it's gonna help me
03:50sketch this graph definitely. So
03:52what I'm looking for is
03:54this is like my think
03:56of these as my x
03:57values and I'm looking for
03:58my key values so are
04:00the key points the key
04:02points are the x intercept
04:05so this is going to
04:06be one at one so
04:08let's say this is one
04:09and then these are some
04:12total
04:12Let's, our x equals three,
04:15our x equals minus two.
04:16So let's go one, two,
04:18three. This is three, and
04:20I want zero negative one,
04:24negative two. So something like
04:26this. Now, at negative two
04:31and three, we have asymptotes.
04:34So I'm just gonna put
04:35a dot, dot, dot, dot.
04:37That is my vertical asymptote,
04:40dot dot dot. And at
04:42one, I have zero. So
04:45what I'm kind of, what
04:46I'm doing with a sine
04:47diagram is I'm writing is
04:51the function positive or negative
04:53or zero or an asymptote.
04:55So here it's an asymptote
04:57zero and asymptote. I'm more
04:59interested though, is it positive
05:01or negative? Because I've already
05:02got these. So when the
05:05function is less than negative
05:07two,
05:08Is this function positive or
05:14negative? Now you can think
05:15of it as well, let's
05:18think of a value and
05:18you could choose any value
05:20less than negative 2 and
05:21it will work, but you
05:22might as well if it's
05:23saying any value less than
05:24negative 2 and it's not
05:25going to change. It's not
05:26because there's no other kind
05:27of key point here. It's
05:28not going to change anywhere
05:30below negative 2. So I
05:32might as well pick negative
05:33100 or even negative 1
05:34million, but let's go with
05:35negative 100. If I have
05:36of negative 100. So if
05:42I sub negative 100 in
05:43here, I'm going to have
05:44the numerator is going to
05:45be a negative value. And
05:47then the denominator is going
05:49to be negative 100 squared,
05:51which is 10 ,000, and
05:52then plus 100. Well, but
05:54I don't even have to
05:54do the calculation. I know
05:55that the numerator is going
05:56to be negative, and the
05:58denominator is going to be
05:59positive. So if I get
06:00a negative over a positive,
06:03I get a negative. So
06:04everything
06:04less the function for x
06:08values less than negative 2
06:10is going to be negative
06:11and I put a minus
06:13sign or a negative sign.
06:16Between negative 2 and 1,
06:20what's it going to be?
06:21Well, the easiest thing to
06:22put in here is 0
06:23because 0 is pretty negative
06:252 and 1. If I
06:27put in 0, I get
06:281 sixth. So here it's
06:30going to be positive
06:33of between 1 and 3.
06:37What's it going to be?
06:38Well, let's put in 2.
06:422 is value to put
06:43in there. If I put
06:45in 2, I get a
06:48positive on the numerator. 2
06:50minus 1 is 1. So,
06:51a positive on the numerator.
06:53And then 2 squared is
06:544, 4 minus 2 is
06:552 minus 4 is negative
06:574, negative. So, I have
06:58a positive over negative. So,
07:00this is negative.
07:01and then if I put
07:02in a number bigger than
07:04three, let's go with again
07:05a hundred. I'm gonna have
07:07a positive over a positive,
07:10definitely, so this is going
07:11to be positive. So that's
07:14my signed diagram. Now you're
07:16gonna see where this is
07:16very, very useful as I
07:19draw my graph. Okay, so
07:23let's get out. Let's get
07:27out a set of
07:29axes here. Okay, now I
07:37know I have, there's some
07:42key points here happening at
07:43negative two, one, three. So
07:45let's put in my asymptotes
07:47first. So I'm gonna put
07:49in, I'm gonna put in,
07:51I'm gonna use different colors
07:53obviously, you don't need them,
07:54I just use dotted lines
07:55if you want, but for
07:56Nate I'm gonna put
07:57negative two, a vertical asymptote
08:01there. And the three, a
08:04vertical asymptote. So that's negative
08:06two, one, two, three. Let's
08:09go here. Doesn't have to
08:11be perfectly accurate, but this
08:14is negative two, and this
08:17is three. And at one
08:20here, it's gonna cross the
08:25the x -axis. So let's
08:29try and figure out what's
08:30going on here. So we
08:33know there is a horizontal
08:37asymptote at zero, so here.
08:40And we know there's a
08:41vertical asymptote at negative two,
08:43and we know that the
08:44function is always negative anywhere
08:46less than negative two. So
08:48what's gonna happen is, look,
08:50I have a horizontal asymptote
08:51here.
08:53And a vertical asymptote here.
08:56So this is going to
08:57approach this, approach this, and
08:59it's always negative. Fine. Next
09:02bit. Between negative two and
09:05one. So between negative two
09:07and one, it is positive.
09:11It's also, I should have
09:12put in, it's going to
09:12cross the y -axis at
09:15one sixth. These don't have
09:17to be the x and
09:19y axis kind of
09:21different scales but whatever. So
09:23this is going to be
09:24this is one sixth. So
09:27it's positive actually I don't
09:30think that's going to work
09:31too nicely. Let's put that
09:33in after right but it's
09:36between negative two and one
09:36it's positive and it's going
09:38to touch the x axis
09:40at one and then it's
09:42going to go negative and
09:44it has a an asymptote
09:46here. So what's actually happening
09:47is it's positive so it
09:49has to
09:49approach this asymptote. And this
09:52is not drawn very well.
09:54It has to approach that
09:55asymptote there. And then it
09:58has to do this. Sorry
10:00guys, this I just cannot
10:02accept that. Okay, so something
10:06like this. Now here's my
10:08asymptote. Here is my asymptote
10:12there, whatever. It's going to
10:15touch go through and here's
10:17my one sixth goes through
10:19one sixth it goes through
10:21one and then from one
10:23to three it's negative but
10:25again I have this vertical
10:27as much as gonna have
10:28to come down like this
10:29and do something like this
10:33select that is not a
10:34great drawing but whatever you
10:35get the point just come
10:36down here through one down
10:39like that so here it's
10:41negative fine and then after
10:43three it's positive and again
10:45And I have my vertical
10:47estimate out here and my
10:48horizontal estimate out here is
10:49just gonna look a bit
10:50like this, but in the
10:51positives it's gonna look like
10:53this. Okay, that's it drawn.
11:00That is this function, x
11:02minus one over x squared
11:04minus x minus six. Now,
11:08if, I mean, depending on
11:10the question, if he says
11:12indicate
11:13equations of vertical horizontal asymptotes
11:17will then write down here
11:18at the side what your
11:19horizontal asymptotes are. Or in
11:21your vertical asymptotes, or you
11:22can write here x equals
11:24negative two, x equals three,
11:27you can even put here
11:28y equals zero. And again,
11:30like you've shown, I mean
11:32you will show your working
11:35here. If I hadn't put
11:36that I'd have put number
11:37three horizontal asymptote by equals
11:39zero. Okay, I want
11:41want to just show you
11:45on Desmos what's going on
11:48here. So this is basically
11:51what I've drawn. We have
11:53our vertical estimate of negative
11:54two and the three. It's
11:57come down here through a
11:59sixth. That will be one
12:01sixth and goes into the
12:03negative. So it's obviously a
12:05better version of what I
12:06drew. There are different, this
12:09can look
12:09different ways. So like here
12:13for example it went from
12:14the it went from the
12:16positive into the negative but
12:17it's possible for that to
12:20look these generally look depending
12:23on how things depending on
12:24what these these letters are
12:27and I've added these sliders
12:28here just to change things
12:29and show you the transformations
12:30but I mean this doesn't
12:33have to go back into
12:34the negative so depending on
12:36I mean obviously
12:38calculating those equations of asymptotes
12:40and things change as you
12:41go through here, but these
12:43graphs can look slightly different.
12:46Something like this, it doesn't
12:47have a vertical asymptote because,
12:49well, if you did the
12:52math, the denominator doesn't have,
12:55there are no solutions for
12:56this to equal zero. So
12:58let's put that back to,
13:01sorry, this was one. So
13:03you can change, I recommend
13:05you actually
13:06going around and playing with
13:07these just to see the
13:10different types that you can
13:12get. But the two, the
13:13two usual ones are the
13:14ones that I drew and
13:16the ones like this where
13:18instead of going down into
13:19the negative, it comes back
13:20up. Now exactly once this
13:21minimum point, well, until you
13:24study calculus, you won't be
13:26able to get that, unless
13:28obviously you have a calculator.
13:29And that's the final thing
13:31I want to say is
13:32obviously if this is a
13:34paper
13:34per 2 and you have
13:34a calculator, you can just
13:37sketch it or you can
13:39just draw it yourself. What
13:41the calculator are used the
13:42calculator to solve any questions
13:46or solve any equations. Okay,
13:49that's it in the next
13:51lesson. I'll do the next
13:53type where I have to
13:54quadratic in the numerator and
13:55the linear function on as
13:57the denominator.