00:00Hi guys. So in this
00:03lesson, we're going to classify
00:04stationary points. Now, what I
00:07mean by classifying a stationary
00:08point is decide whether it
00:10is a maximum, a minimum,
00:12or a point of inflection.
00:13So a stationary point is
00:15either this maximum, a minimum,
00:18or a point of inflection.
00:19It is where the first
00:20derivative equals zero. So the
00:22first derivative of zero, first
00:23derivative of zero. But I
00:26don't know, like just from
00:28looking
00:28it's fine and classified any
00:29stationary points on the curve
00:31y equals e to the
00:32x minus 3x without a
00:34calculator. So I don't even,
00:36I don't know just from
00:38looking at that if it
00:39even has a stationary point
00:40and if it does I
00:41don't know what type of
00:42stationary point it is because
00:43I don't know how to
00:44just graph that. I kind
00:46of just graph it like
00:48I could graph it quadratic.
00:51So let's find out. So
00:53the first thing is we
00:54need to find the stationary
00:55point. The way
00:56we find the state tree
00:57point is we find the
00:59first derivative, we find where
01:00the first derivative equals zero.
01:03So, first derivative equals, this
01:08is quite an easy one
01:09because the first derivative of
01:10e to the x is
01:11e to the x and
01:12the first derivative of this
01:13is negative three. At stationary
01:18point, dy dx equals zero.
01:24Therefore, E to the x
01:30minus 3 equals 0, E
01:34to the x equals 3,
01:36and then hopefully you'll notice
01:37all of this, x equals
01:39l and a 3. Okay,
01:43that's the x coordinate. So,
01:45E says find and classify
01:46any stationary points, so I
01:47need to find the coordinate.
01:51This is not the
01:52the coordinate, this is just
01:53the x -coordinate. So I
01:55need to find, let's do
01:57it up here, at x
02:01equals ln of three, y
02:04equals e to the power
02:07of ln of three minus
02:09three times ln of three,
02:12which is three ln of
02:13three. So I've just subbed
02:15ln of three into this
02:16to get me y. e
02:19to the ln of three
02:20Again, hopefully you know that.
02:22If not, re -watch my
02:25exponents lesson or log lesson.
02:30So the answer to this
02:31is three, because the E
02:33and the L and kind
02:34of the kind of cancel
02:35each other out. And then
02:36three minus L and of
02:38three. Therefore, the stationary point,
02:44there's only one of them,
02:45because there's only one solution
02:46to this. The stationary point
02:47is
02:48ln of three, comma three
02:51minus three, ln of three,
02:54lots of three's there, but
02:55that's the stationary point. Now,
02:58to classify, to classify the
03:01stationary point, I need to
03:03find the secondary derivative because
03:04that's key here. So here,
03:07at a maximum, the second
03:08derivative is less than zero.
03:10At a minimum, the secondary
03:11derivative is greater than zero.
03:14But be careful with this
03:15one. At a point of
03:16infinity,
03:16reflection at a point of
03:20inflection the second derivative is
03:23equal to zero. So let
03:24me actually write this down.
03:25This is important. At a
03:28point of inflection at a
03:31point of reflection d2y dx
03:36squared equals zero. Always. Point
03:40of inflection, second derivative is
03:42zero. Yes.
03:44If d2y squared d2y dx
03:54squared, if I don't have
03:56to talk, but if d2y
03:58dx squared equals zero, that
04:01doesn't necessarily mean it's a
04:03point of infection. Not necessarily
04:08Poy.
04:12So just be careful. At
04:14a POI, the second derivative
04:16is always equal to zero.
04:18But if the second derivative
04:19equals zero is not necessarily
04:20a point of a reflection,
04:21the next example I'll actually
04:23show you how to find
04:24that what it is then,
04:25but just to note this,
04:28right? Okay, so I have
04:31the stationary point. To classify
04:33the stationary point, I'm actually
04:37going to need a little
04:38bit more space here, so
04:40I'm going to just
04:40move this down to here.
04:47Okay, to classify, I need
04:52the second derivative to classify,
04:59classify, we need d2i.
05:08Okay, so I need to
05:10get the second derivative d2
05:12y dx squared is equal
05:16to this is the first
05:18derivative e to the x
05:19minus three so the second
05:20derivative is just e to
05:22the x because there you
05:24go it's e to the
05:25x and then the derivative
05:26of three is just zero
05:29now I Want the second
05:33derivative at this point so
05:36d2y d2y dx squared at
05:42x equals ln of 3.
05:47That's the fancier way of
05:47writing d2y dx squared at
05:49x equals ln of 3.
05:50If I was using the
05:51f dash dash notation, I
05:54could just say f dash
05:55dash of ln of 3.
05:57But anyway, that equals e
06:01to the power of ln
06:03of 3. And once
06:04e to the power of
06:051 of 3. Well, it's
06:073. Therefore, as d2y dx
06:18squared, let's continue with at
06:22x equals 1 of 3
06:25is greater than 0. So
06:28this is greater than 0.
06:303 is obviously greater than
06:310.
06:33Um, stay -stream point is,
06:41uh, minimum. At the minimum,
06:48the second derivative is greater
06:50than zero. If the second
06:51derivative is greater than zero,
06:53you know it's a minimum.
06:54If the second derivative is
06:55less than zero, you know
06:56it's a maximum. As I've
06:58said, if you know the
07:00second derivative equals
07:01to zero, we need another
07:04strategy. What I'm going to
07:05show you how to do
07:06it here. Okay, so finding
07:09classify the stationary point on
07:12the curve y equals x
07:14to the force. So this
07:14is kind of a classic
07:15example where the second derivative
07:17is going to equal zero.
07:19And let me show you.
07:20In fact, if you actually,
07:21you might actually know what
07:23this curve looks like. Y
07:25equals x to the 4.
07:27It's basically, it's basically
07:29basically just a kind of
07:31steep y equals x squared
07:34curve. So it actually looks,
07:37it just looks like this.
07:40Because all the, at one,
07:41one to the power four
07:42is one. Two to the
07:43power four is 16, three
07:45to the power four is
07:46positive. Negative two to the
07:48power four is 16. So
07:49it's just like a steep
07:50quadratic if you like. So
07:52I'm gonna go through, I'm
07:54gonna go through that procedure
07:56again.
07:57I'll go through it pretty
07:58quickly. So why is this?
08:01So dy dx equals 4x
08:05cubed. 4x cubed equals 0
08:10for the stationary point, which
08:11means x cubed equals 0
08:14or x equals 0. So
08:15the only solution to this
08:17equaling 0 is 0. So
08:19the stationary point is at
08:200 and then y
08:25Well, at x equals 0,
08:29y equals 0, because y
08:31is x to the 4.
08:32So the stationary point is
08:340, 0, 0, 0, which
08:38we expected, because that's it
08:40right there. Okay, now let's
08:43get the second derivative, d2y
08:46dx squared equals 3 times
08:494 is 12, x squared
08:52bring down the power of
08:53y.
08:531. Fine. At, so d2i
08:58dx squared at x equals
09:030 equals 12 times 0,
09:08which is 0. Now, so
09:10I have the second derivative
09:12is equal to 0. This
09:14is important. I said, if
09:16the second derivative is 0,
09:19we cannot say it's a
09:21point of effect.
09:21because this is a perfect
09:23example where it's not a
09:25point of reflection. So what
09:26we have to do is
09:27we have to make a
09:29table and what we're going
09:31to do with the table
09:32is we're going to say
09:34x here and I'm going
09:37to say f dash of
09:39x here and we're going
09:43to look left and we're
09:48going to look right
09:49of zero. Now what I
09:51mean by that is this.
09:56So it's all happening at
09:58this. This is my stationary
10:02point right here. But I
10:04want to know is this
10:05a max or a mean.
10:06So I'm going to look
10:07at zero. That's where it's
10:12this is my stationary point.
10:15And I know the derivative
10:17derivative at my station point
10:19is zero. Fine. But I'm
10:22going to look to the
10:23left of it. I'm going
10:25to look to the left
10:26of zero. And I'm going
10:29to look to the right
10:31of zero. So what I
10:34mean by minus is just
10:36slightly to the left of
10:37zero and slightly to the
10:40right of zero. Now we
10:40don't need to get too
10:41technically here, but it could
10:43be negative zero point zero
10:45zero
10:450, 0, 0, 0, 1,
10:47and this could be 0
10:48.0, 0, 0, 0, 0,
10:500, 0, 1. So I
10:51could get into limits and
10:52things of that, but I'm
10:53not going to bother. Because
10:56it's quite straightforward. If you
10:57just think of to the
10:58left of 0 and to
10:59the right of 0. Now,
11:02dy dx dy dx is
11:064x cubed. So f dash
11:10of x, let me write
11:11it down here. f dash
11:13of x.
11:13equals 4x cubed. Actually maybe
11:20minus is wrong, the wrong
11:22thing to use there. Sorry.
11:24I'm actually gonna write, I'm
11:26actually gonna write left and
11:31right. Okay, so slightly to
11:35the left of x and
11:37slightly to the right of
11:38x or slightly to the
11:39left of zero and slightly
11:40to the right of zero.
11:41Now, if f dash of
11:44x is 4x cubed, what
11:46is the gradient slightly to
11:50the left of 0? So
11:52that would be a very
11:53small negative number. If you
11:54want to think of it
11:55as negative 0 .1 or
11:57negative 0 .001, well when
12:00I cube that negative number,
12:03it's going to be negative.
12:05So if I put a
12:08negative number, a small
12:09negative number into this, I
12:13get a negative f dash
12:17of x. And when I
12:19put a positive number in
12:20here, I get a positive
12:24gradient. And what happens is
12:27I get a negative gradient
12:33that looks like this. Then
12:36I get a zero gradient
12:37that looks like this, and
12:39then I get a positive
12:40gradient that looks like this.
12:42Make it from negative to
12:44zero to positive has to
12:46be a minimum. So after
12:50doing this, I can say
12:51therefore, therefore, stationary point is
13:00a min -a -mum.
13:06Okay, this question is obviously
13:10I'm confusing and it gets
13:12a little bit technical here
13:13I do want you to
13:14know how to work out
13:15this table you will use
13:16it when the second derivative
13:19equals 0 because you cannot
13:22assume clearly that it's a
13:24point to infection if the
13:25second derivative is greater than
13:270 Then it's a minimum
13:29and if it's less than
13:310, it's a maximum. It's
13:33only when it's equal to
13:34that you are not sure.
13:37Okay, see you in the
13:38next lesson.