00:00Hi guys, so in this
00:01lesson we're gonna find the
00:02vector equation of a plane.
00:04So this is where things
00:05get really really interesting and
00:07Perhaps a little bit complicated,
00:09but hopefully it's not that
00:10bad. So Firstly, what is
00:12a plane? Well a plane
00:14this is gonna my attempt
00:15to draw a plane a
00:16plane is a flat surface
00:18thing of does a flat
00:19surface like you're like a
00:21wall is a plane or
00:22the ceiling or the floor
00:24as long as it's flat
00:26in
00:28Geodebra, it looks something like
00:30this. This is a plane
00:31and a bit like a
00:32straight line this kind of
00:35goes on forever. If I
00:36zoom out, it's always there.
00:39Soon back in again. So
00:40this is a plane. Okay,
00:43we want to, we want
00:47to find the equation of
00:48a plane. So we're going
00:49to focus on this one
00:51here. And this lesson in
00:52the next two lessons, I'll
00:54talk about these two.
00:56So essentially what we need
00:57to do a bit like
00:59a straight line, we need
01:00to find an equation that
01:02will allow us to get
01:03to every single point on
01:05the plane. Now let's start
01:07at zero. Remember for the
01:09equation of a straight line,
01:10we started at zero and
01:12we got to a point
01:13on the line. That's the
01:15same with the equation of
01:16a plane. A is the
01:17position vector of a point
01:19on the line. So A
01:19is on the plane. So
01:21A is on the plane.
01:23And with the straight
01:24line what we did is
01:25we got onto the line
01:26and we moved along the
01:28direction vector which in this
01:29case would be B as
01:31far or as short as
01:34we wanted so that we
01:36got all the points and
01:37we had that's why we
01:38did lambda B lambda was
01:39the parameter you got all
01:41the points of the line
01:41and we could go this
01:42way and get all the
01:43points of the line. Now
01:45the plane is similar but
01:47if you just have one
01:48kind of direction vector like
01:50this you can only
01:52only get to a straight
01:53line. So you need two.
01:55And importantly, the two cannot
01:57be parallel because if C
01:59was parallel to B, again,
02:01you just end up going
02:02this way in a straight
02:03line and you wouldn't be
02:04able to get onto the
02:05plane. So if C is
02:07not parallel to B, and
02:09by the way, both vectors
02:10have to be parallel to
02:11the plane. Now what I
02:13mean by that is this,
02:15let's have a look at
02:16Georgia Brigan. Here I've drawn
02:18a vector that's parallel to
02:20the
02:20So I want you to
02:21see it. Like this, maybe
02:23it's not as easy to
02:24see. Let's look at it
02:25from above. So there's my
02:28plane, this kind of straight
02:29line. I'm directly above it.
02:30And this vector is parallel
02:32to the plane. There you
02:35can see it again. It
02:39is parallel. Let me stop
02:43there. Hopefully you can see
02:46what I mean by parallel.
02:47So you'd have this one.
02:48and another vector also parallel
02:50to the plane but not
02:50parallel to each other. Okay,
02:54once I have the two
02:55vectors like this that are
02:56parallel to the plane but
02:57not parallel to each other,
02:58I can get to any
02:59point on the plane and
03:00that's the point, forgive the
03:03point, that is the point
03:04of getting of the equation.
03:06So I can get to
03:07imagine I wanted to get
03:08to this point here. I
03:09go along B, however many
03:13I need to do, and
03:15then up C, however
03:16remaining I need to do.
03:19And the lambda and the
03:20mu will decide how much
03:23b's I need to add
03:24and how much c's I
03:25need to add to get
03:26to that particular point. Okay,
03:29hopefully that makes sense. So
03:32what we're going to do,
03:32I'm going to do two
03:33examples. One, we have three
03:36points. This is actually the
03:37plane that I've drawn in
03:38geodebar, but we have three
03:39points and I want to
03:40get the equation of the
03:41plane. And then the second
03:42one, I've given you a
03:43line and
03:44and a point and the
03:46same thing we want to
03:46get the vector equation of
03:48the plane. So what do
03:50we do? We need a
03:53point and two direction vectors
03:57that are parallel to the
03:58plane. The point is easy
04:00because I have three. I
04:01have A, B and C.
04:02I can choose any of
04:03them. Let's choose A. The
04:06common mistake is silly mistake
04:07I would argue to do
04:09or to make is to
04:11say, well, this is one
04:11of the, this is
04:12vector B and this is
04:13vector C, but that's just
04:15not the case. This is
04:16a point and this is
04:17a point and they're not
04:17vectors and the position, if
04:19you want to the position
04:20vector of B, let's say
04:21B is somewhere on the
04:23plane, then the position vector
04:25is this and this isn't
04:27parallel to the plane. So,
04:29well, unless the plane went
04:30through zero, zero, zero, but
04:33there's nothing that says it
04:34does. So, that's not a
04:36vector, the position vector of
04:37B or C is not
04:39a direction vector that I
04:40can use.
04:40However, if A, B, and
04:42C, let's say that's A,
04:43that's B, and that's C,
04:44and they're on the plane,
04:48well then, any of A,
04:51B, let's say that's A,
04:54B, and C, any of
04:55A, B, B, C, or
04:58C, A, or the opposite
05:00way, you can do A,
05:00C, C, B, or B,
05:02A, whatever, any of those
05:04vectors are parallel to the
05:08plane.
05:08because they're literally on the
05:10plane. Okay, so let's do
05:13that. Let's find either A,
05:16B, B, C, R, A,
05:18C, or any version of
05:20them. So let's go, I
05:22think I'm gonna go with,
05:25I'm gonna go B, A.
05:28Now again, I could have
05:29done A, B, I'm not
05:30gonna do B, A because
05:31that's A minus B and
05:33I think it just looks
05:33a little bit nicer.
05:36minus 1, 3. So, bA
05:39is a minus b. Again,
05:41I've said this to you
05:42many, many times. That is
05:44an important rule and it
05:45comes up all the time.
05:46So, bA minus b, this
05:48is 1, 1, 2. This
05:50gives me 3 minus 1
05:51is 2, minus 1, minus
05:531 is negative 2, and
05:543 minus 2 is 1.
05:56There we go. That's the
05:58first vector. The next one,
06:00let's go, bC. B,
06:04See, so the reason I,
06:06the only reason I've chosen
06:08BC is because these look
06:09like nice numbers that I
06:10can subtract. But, as I
06:13say, any of them would
06:14have done. So, 4 minus
06:161, 2, because BC is
06:18C minus B. So, 4
06:19minus 1, 2 minus 1,
06:221, 1, 2. And this
06:24equals 4 minus 1 is
06:263, minus 1 minus 1
06:27is negative 2. And 2
06:29minus 2 is 0. So,
06:32these are my 2
06:33direction vectors if you like,
06:36B and C. Therefore, equation
06:41is, or equals. Position vector
06:48of a point on the
06:48line, let's go with A,
06:50but again, I could choose
06:51any of them. Three, negative
06:53one, three, plus lambda, lambda
06:58times this vector,
07:012 negative 2 1 plus
07:04mu times, let me try
07:07that again, mu times 3
07:13negative 2 is 0, 3
07:15negative 2, 0, done. That's
07:20the first one. So if
07:22I have three points on
07:23a plane, I can find
07:25the vector equation of the
07:27plane, provided the 3 point
07:29It's earned in a straight
07:30line because if they're in
07:31a straight line, I'm going
07:32to have that problem like
07:33A, I'm going to have
07:34this, this, and I'm going
07:35to, the only vectors I
07:36get, the only vectors I
07:38can get parallel to the
07:39plane are all parallel to
07:40each other and end up
07:42just giving me this straight
07:43line. So I need three
07:45points, one of which is
07:48not on the same line.
07:52That brings me to my
07:52next example where I have
07:55a point and a line.
07:57So if this point is
07:59on the line, I won't
08:00be able to find the
08:01vector equation of the plane.
08:04Is this point on the
08:05line? No, it's not on
08:08the line because in order
08:14to get this point, t
08:18would have to be 1
08:20to get me 3 at
08:22the bottom. So 1 plus
08:232 gives me 3. If
08:24t is 1,
08:25five plus one equals six,
08:27which is four, so that's
08:28no good. So at this
08:29point is not on the
08:30line. Anyway, it says find
08:33a vector equation of the
08:34plan of the plane containing
08:35the point this and the
08:36line this fine. So I
08:39need, so let's remind ourselves
08:40it's our equals a plus
08:43lambda b plus mu times
08:46c. So I need an
08:48a, I need a b,
08:51and I need a c.
08:52Well,
08:53I have an A because
08:57the point, the plane contains
09:00the point P. So I
09:01have a, it's the position
09:03vector of P. I actually
09:05have a B because this,
09:09if a line is on,
09:12so let's imagine this is
09:13our plane here. Let's just
09:18say something like this. If
09:19the line is on the
09:20plane,
09:21So let's say this line
09:22is going. The line is
09:25actually on the plane. Then
09:28the direction vector of the
09:30line, like so. This is
09:36the direction vector of the
09:37line. This is parallel to
09:39the plane. And also this
09:41point has to be on
09:43the, this point has to
09:45be on the plane as
09:48well.
09:49because the line is on
09:50the plane, so this point
09:51is on the plane or
09:52this position vector. This is
09:55the position vector off the
09:57point, technically it's the way
09:58to say it. Okay, so
10:00I have a point, this,
10:03or I could use this,
10:03but I have a point,
10:04I have a direction vector,
10:05the only thing I need
10:06is my second direction vector.
10:09And how do I find
10:10that? Well, P is on
10:11the plane, let's see, that's
10:13P. And this point here,
10:17This is on the plane,
10:19let's say it's there. So
10:24if I find, and let's
10:24call this, I don't actually
10:28want to call it A.
10:29Let's just call this, I
10:32don't know, let's call it
10:34M. So this is the
10:36position vector of M, I
10:38don't know why it shows
10:38M, but whatever. So if
10:41I get the direction vector
10:43PM, PM,
10:45That will give me pm
10:49would give me another vector
10:51that is parallel to the
10:54plane, which is what I
10:54need. So let's find this.
10:55It's m minus p. This
10:56is m. So it's minus
10:593, 5, 1 minus p,
11:02which is this, minus 1,
11:074, 3. Subtract them. Minus
11:103 plus 1 is negative
11:112. 5 minus 4 is
11:131.
11:131, 1 minus 3 is
11:15negative 2 like this. This
11:18is pm. Now I have
11:21everything. I have everything I
11:23need. Therefore, or equals point.
11:28Let's go with p minus
11:301, 4, 3 plus lambda
11:36times b. Let's go with
11:39this direction vector.
11:41for 1, 2 and then
11:44plus mu times this one
11:492, 1, negative 2. Okay,
11:54there we go, that's it.
11:57So, um, to find this
11:59vector equation of the plane
12:00in this form, I need,
12:02I need the, a point
12:05on the, I need a
12:05point on the plane and
12:07I need 2,
12:09vectors parallel to the plane
12:13but not parallel to each
12:17other. Hope that makes sense.
12:19In the next lessons we're
12:20going to look at these
12:22other forms of equations of
12:24a plane. This one is
12:26by far, in my opinion,
12:29the best. It's my favorite
12:30one to work with. I
12:32would imagine most people will
12:35say it's their favorite one
12:35to work with. It's certainly
12:37the most kind of concise
12:39if you just type in
12:41a explicitly right plus cz
12:43equals d where those are
12:45numbers into geodebra you'll get
12:47yourself a nice plane and
12:49I'll explain what all that
12:49means in that lesson. Okay
12:51see you then and hope
12:53that all makes sense.